Is this why you are so much faster? You're running the wide wheel/offset spec while I'm running the dealership spec?
I'll bite, because there isn't enough talk about the little things.
The difference in lap times between different size wheels and tyres varies by car, and by track. It depends upon many factors including the balance of the car in stock form, its individual strengths and weaknesses, and suspension design and clearance. IMO, they did a good job with this stuff in the game, they just did a terrible job of explaining it to the players. Sort of how after all this time most don't understand the effects of the different settings of grip reduction off the racing line, slipstream, tyre wear multiplier, etc.
Anyway, depending upon the car you can find small gains in changing wheel size, just like other modifications. What is unique about the wheels, however, is that they are allowed in a tuning prohibited room, wheras most other modifications are not. Some cars struggle to put down power and seemingly benefit from smaller wheels. Other cars can change balance and breakaway characteristics with track width changes from wider wheels and offset. Some get worse. For example, IMO the DB5 is a little more stable with aftermarket wheels. That introduced understeer at Brands Hatch in places the stock wheeled car did not experience, and as such I was a tenth or two slower in the car with aftermarket wheels. And some, apparently like the Mangusta, become undriveable.
Some cars the difference can be greater, but to answer the above question the difference is rarely more than a few tenths at most. In extremely unique situations there may be a little more time, but I've never personally experienced multiple seconds between wheels on a stock street car. I also haven't tested every car and every track or anywhere near that.
While in GT7 they did close a door with regards to brake bias in spec racing, but they did open a window with wheel options, and (I still need to test this but I have a hunch) roll cages. I just don't spend as much time driving as I used to, and spend pretty much zero time testing since I retired from competition a decade ago, so I haven't mastered all the little details. Back when I had more time and this place was a little more hoppin, these are exactly the types of things I used to go over when I would do a track walk in a practice room.
Practice together, share info. You'll all get quicker, faster.
Trying to maintain any type of speed there and the car behaves unrealistic on the jump. Wants to do a barrel roll! 
Only way I've been able to take it successfully is to tap the brakes before takeoff. Anyone have a better method? Was running 50/50 split with brakes at -4 to front.
