Wider tire slows the car right?

yea after a certain width you're just making it harder on your car

I and many others run 245/40/19 all the way around...performs great on the sedan at least

Depends on the situation and what you mean by slows the car. Wider tire has more friction and weight which can mean a lot more grip to put down power or handle OR it can just mean using more gas to go the same speed.

245/35 sidewall height is only 0.15" less than 225/40. I run 245/35 front and 255/35 rear on my sedan; ride is perhaps a little firmer but no issues.

Traction...
The factory wheels are only so wide which limits size of tires you can put on them. Since most turbo kits can easily hit 400whp/400wtq, trying to prevent the wheels from spinning all the time with 255 street tires isn't going to happen.

I thought rears had to be 255/40/19 with fronts 245/35/19?

Is it bad to go 255/35/19 in the rears? Would love to keep the same ratio front and back

Not correct. I went with a popular setup of 245/35/19 & 255/35/19..this stay's within the vdc's limits.

^ Ive never heard/read of that setup

255/35/19 is 2.6% too fast compared to stock

255/40/19 is 1.2% too slow compared to stock

Seems like the 255/40 is a closer match tho

I'm running 225/245 35 with no issues, looks better too..

It's up to you.

Is there a big difference in looks? Do you have any pics of your car?

Yeah it's a big difference, no pics though, sorry..

245x35x19 & 255x35x19. No issues with the VDC and there's many people running that setup.

Biggest is go would be 235/255..