Seeing as the offsets of the front and rear tires are different, it's kind of hard to tell exactly what wheels will fit on the Crossfire. Judging by what I can see through depth perception, I would say something like this would be the best combination to really fill out the wheel wells without causing the fenders to rub:

30mm spacers - front
40mm spacers - rear

19x9 inch wheels - front
20x10.5 inch wheels - rear

240/30ZR19 tires - front
275/40ZR20 tires - rear

Those dimensions would be using wheels with no offset. If you used offset rear wheels like the stock rims you could compensate by dropping down to 30mm spacers in the rear. A couple of other things to consider being the fact that finding rims in 20x10.5 would cost a small fortune (probably would need to be custom made), and the sidewall depth of the tires would be iffy. You could get 20x10's, but your rear tire size would suffer. Going with 40ZR20's in the rear would also mean you really couldn't carry much in the way of cargo, and even without the additional weight you'd need to watch yourself on rough and uneven roads.

Having said that, going with 20 inch wheels both in front and in back would drastically change the handling characteristics of the car, and not necessarily in a good way. In order to get a 20 inch rim to fit up front you'd need to use an exceptionally thin side walled tire. I wouldn't recommend it for two reasons. Number one being that you'd be looking at some ridiculously shallow tread depth, meaning your tires wouldn't last long, and number two being you would magnify the cars already induced understeer.