Testarossa doesn't have any modifications on this picture, this is a stock front lip, just painted
Yeah, I'd be somewhat surprised if we get to add any aero parts to the Ferraris. But I think I could still have fun changing the paint, adding decals, and changing out the wheels. If I'm wrong, though, and we do indeed get some aero parts for even one Ferrari, I'd be real happy. Heck, I don't even know how much we'll be able to change the internal bits on those things. Not that I'd mind either way - I plan to get at least two of every car, with one to leave as-stock, and another to modify however I'd like. And if we can't do much with the Ferrari models, then that's fine by me, because we'll still have plenty of ways to modify seemingly every other car in the game.
At the very least, I plan to potentially equip the Azzurro Met. paint from the 458 Italia on some Ferrari models that don't offer it. It's probably my favorite Ferrari paint that isn't the usual Rosso Corsa/Rosso Scuderia/Nero Daytona/Giallo Modena.
We can also clearly see from recent gameplay footage that, at the least, we can really tinker around with a Lamborghini Aventador.
EDIT: As for wheels, I could say quite a bit on what wheels I plan to equip to what vehicle. Generally, I plan to go for designs that are similar to the stock design. So if a car comes with a center-lock design as standard, then I'll pick a different center-lock design to compliment it. This would mostly apply to various higher-performance cars like the McLaren F1, but also the Gr.3 cars. Same thing with cars that have wheels that have four lugs, which are generally smaller cars, like the kei cars or the 2008 Fiat 500. And finally, I plan to pick between the six-lug Enkei Baja or any KMC design for any relatively new trucks/SUVs we get, like the Toyota Tundra TRD Pro and the Ford F-150 SVT Raptor, but maybe not so much stuff like the classic Jeep we're getting.
I don't see myself modifying the race cars in general, though, as not only do I think they're fine as-stock, but I don't even know what options we'll even have for the cars in Gr.3/Gr.2/Gr.1/Gr.B, though I could see there potentially being more options for the Gr.4 machines, especially those that are made-for-game models like the Ferrari 458 Gr.4. I just don't think it'll be necessary to modify the race cars since I'll want them to still be capable of complying with any regulations I may see, whether it's in the campaign or online.
EDIT2: Plus there's the factor that some race cars may have unusual wheel sets, like the differently-colored wheels on the Hyundai Genesis Gr.3, though it'd be really cool if we could somehow replicate that this time around in GT7's version of the livery editor.