Not only do you still drive, but you autocross??? RESPECT!
You can do software only PPK if you have S840 which most all sport package 335i's have. But I don't know if you can get the full cooling upgrade kit separately. I thought I'd order both (about $1200.- from Tischer).
N54 or N55 does not matter AFAIK in terms of oil/coolant temps. 300 HP and two turbos Vs. 300 HP and one twin-scroll turbo... it's a wash. The oil cooler is too small for either engine.
What does matter is the auto tranny. It uses a oil/coolant heat exchanger and dumps it's heat into the cooling system. I'm sure you'll be OK for autocross, but on a road course you'd need the extra cooling. See the epic "N54 Limp Mode Party at Track" thread for more info. At the very least run the smallest amount of coolant mix you can get away with for your area (but not less than 15% coolant) and a water wetter (RedLine, Motul, HyperLube).
I'm nearly at 4 years / 50k and need to have a cooling flush done. BMW, in their infinite wisdom, say the coolant lasts forever
I figure I can get the PPK installed and they can change the coolant at the same time. And then I have nearly the identical 335is engine.
I've already got one of the 335is approved transmissions (6 speed manual) and I've installed the M/is shifter and the parking brake handle (cheap easy mods). Pedals coming soon. That stuff is all the easy part.
If the PPK doesn't have overboost, then I'm sure one of the tuners will figure out how to enable that feature.