I reckon they'll have to at least put the P1 around to. I'm not sure how they'd explain that on the show if they only had a lap time for the 918 especially since the Jennifer bet thing would still be unsettled. Also, the only reason we know that there was a 918 at the track is because Jeremy posted a thing about the guy's shirt. This doesn't mean that the P1 wasn't also there... Also on iTunes they say that the series will include "Jeremy Clarkson, Richard Hammond, James May and The Stig are back in the all-new 22nd series of the world's favourite car show. The presenters race each other across St Petersburg, embark on an ambitious project to build better ambulances, find the perfect classic convertibles for an English summer, celebrate the world?s most indecisive car company, and pay perilous tribute to the Land Rover Defender. Plus, there are sensational road trips, tyre-smoking track tests, top celebrity guests and the ultimate face-off between the world?s greatest hypercars." Note the plural 'cars'; this could either mean that the LaFerrari review is what they're referring to or (and I think this is more likely) they managed to get a 918 and P1 (and maybe LaFerrari but I doubt it) down to the track. This would also make sense seeing as there's one episode left where we can only speculate what we'll see. We know that in some order over the next three episodes there'll be the classic convertible film (a main feature), the cheap SUV challenge (a main feature), and the rallycross thing with Tanner (could be a main feature but I think probably more likely a smaller feature like the rallycross item in 18x07) so there's one episode that's left unaccounted for. My guess, seeing as Clarkson said he was writing the shootout last year and that it would be a "half hour special", is that it is indeed the hypercar shootout.
In summary, I think it's pretty much a given that we'll see the hypercar shootout in episode 10 as the big finale to the season. And I'd say that since they wouldn't dedicate half of an episode to a test of the 918 by itself - since they already did that in 21x05 - it'll be at least the P1 and the 918 together.