It depends on a few factors for the rubbing on the inside of the coilover collar. Your wheels are 10mm closer to the collar than mine are (about 1/2") and also tire width plays a role as well. The overall diameter of the tire from 17 to 18" are typically not very different from each other, it just depends on what width and profile you run. Mine are about 25.5" overall tire diameter (it's a 255/40/17) which is similar to a 235/40/18 in overall diameter. It also depends on tire manufactures too, as some manufacturers have wider or taller tires compared to other manufacturers even though they are rated the exact same sizes on the side of the tire.
I am definitely leaning toward 3" for the exhaust, as I have a few buddies that have been running K swapped civics for years on the track that swear by 3" exhaust for K series. Makes sense, since the stock downpipe on our cars is pretty big! I just don't want a super buzzy/droning exhaust on my car being that it's a daily driver, and I'm 40 years old and not a kid anymore haha
I know your +35 offset will clear without any problems. I'm more so talking about your buddy with the +48 offset in 17x9 not hitting the collar. Then again heeltoe was running 17x9 +48 offset wheels a couple years ago and he didn't report back any clearance issues either. Must be my 18" wheels.
Also know tire manufacturers don't all run tires to the same spec
size. My tires weren't hitting the collars it was actually the inside lip of my wheels. Ouch.....
Your boys are right 3" exhaust is the way to go on the k-series but trying to find that happy medium with noise is risky...Hahahaha
You're absolutely right the downpipe on the stock header is HUGE!