The 4.0 is lighter than the other 997's and also almost as light as the narrower Mk1 996 GT3. The PSM is just a little annoyance and doesn't add any weight or cost as it's just a string of software which can thankfully be turned off. If they had replaced the mechanical limited slip diff with the PSM like McLaren does then it would be a completely different case.
I don't disagree with any of the above. But none of this supports the 4.0 being the most raw and basic modern street legal Porsche. In fact, things like the dynamic engine mounts, PASM, etc. make it less basic than some other modern Porsches. I'm not saying this to argue that the RS 4.0 isn't desirable, but rather that the view of it as "most raw and basic" isn't actually supported by the facts. The 996 GT3 RS is more basic in equipment and technology and has a more raw feel from behind the wheel.
Still the driveline is what makes the 4.0 stand out for me.
The driveline is definitely the standout component for me as well. It's funny you mention the mechanical LSD - it wears out quickly (this plagues all 997 GT3s, I'm not sure if it impacts the 996s as well). But the internals can be replaced with upgraded components to further improve the car. The 4.0 also has an improved clutch mounting bracket... "Um, what? Who cares?" Well, I do. The bracket in our 997.1 RS broke while the car was being driven on track. Gears would no longer engage, and though the clutch was barely worn, it had to be replaced to replace the mounting bracket. The redesigned 4.0 part doesn't have the weakness the part in the 3.6L and 3.8L cars has. With all the little upgrades like this, plus the added power and the good powerband, the 4.0 is a very desirable car for me.
Adding a late edit because I want to bring the post back around to the 991. It's a bit of a winding path, though.
Each of us has our own preferences for the transmission, and I'm not intending to tell someone their personal preference is wrong. I do, however, think the preferences should be based on the realities of the car. If you'd want a traditional manual in your GT3 because you prefer it, because you'd rather do the shifts yourself, because you enjoy the challenge of pulling off that perfect heel-toe downshift, that's your preference and I won't try to talk you out of it.
I will, however, tell you why I think PDK is more in line with the character of the GT3. I've spent thousands of laps in ours, as well as many street miles. I've also driven a few other GT3s (RS and not, stock and not), so I know the things I'm feeling aren't due to some flaw or uniqueness with our specific GT3. I've also tracked Boxsters and Cayman, and have spent some time in a Cayman R with PDK. To get a feel for it, let me talk you through a section of one of our local tracks. The track is Motorsports Ranch in Cresson, TX. The layout is the 3.1 mile version,
track map here. The section is exit of turn 8 (the hairpin) headed toward turn 9. The track goes uphill and then downhill, here, cresting around the last left before 9, and then going downhill sharply.
For turn 8, that hairpin, the car is down to 2nd gear. There's a crest on the edge of the hairpin that the car hits as a diamond, right front, left front, right rear, left rear. It's hard on the power tracking out of 8, but with a bit of a lift and modulation for that crest. The suspension then has to settle - stop moving around. It does that just in time to turn for that first gentle right. At that point, the car has reached the top of 2nd gear - time for a shift to third. Come off the throttle - that unloads the rear, making the rear suspension unstable again, and loads up the front. Shift complete, and done quickly, back on the throttle. That unloads the front and loads up the rear... now the front suspension has been made unstable and low on grip... but the rear is still trying to stabilize from the shift. And the car needs to be turning left and going over the crest, with unstable suspension. The car is disconnected from the driver, and while it's certainly controllable, it's not as in tune with the driver and not as fun. The crest keeps the car unstable, and the continued turning means the driver doesn't really have a chance to get the suspension stable again until the braking zone for 9, where the combination of the end of a downhill and a brief patch of straight mean the driver can load up all four corners of the car, pushing the suspension back into stability.
With PDK, that shift would be smooth. The suspension wouldn't be unloaded. The car would stay loaded up for the rest of the right and the left going up the hill. The car would behave better going over that crest, which means I could use more throttle on the downhill and still have confidence I could brake sufficiently for 9. The car would be more engaged and engaging. The car would feel better and be more fun. The laps would be faster, not just because of time saved in the shift, but because the car would be more stable and thus would be able to give more pace to the driver.
And I've tried short shifting to third before the right hander. But the car's still unstable from that crest coming out of 8. It's no better - the car still never gets a chance to properly stabilize and it's still disengaged from the driver.
The rest of the car is so on edge, so responsive, so nimble. But the transmission in it is clunky, slow, and relatively imprecise. It's out of character for the car, and it impacts the car's on-track feel in a negative light. Give me an MX-5, and I'll take a traditional manual. Give me a Carrera S, and I'll take a traditional manual. Give me a GT3, and I want PDK. PDK is what fits in and what works in the car. I'm glad the 991 has PDK, particularly since they've improved it. While I have nothing against a manual option, Porsche has chosen not to offer it. I can't tell you why, but I am glad PDK's available and PDK's what they chose if they're not going to offer choice.