Most 4 into 1's are designed for the street any way, you only have to look at the baffle design compared to a race bike, so most 4 into 1 pipes are fine for the street, the bigger baffle gives increased torque or shifts the torque curve back down the rev range, you would barely notice this difference at all using a 4 into 2. I understand the "even" or "Balanced" look a 4 into 2 gives but i wouldn't be reading to much into the "4 into 1 has less torque" thing , it would barely Make any noticeable difference, every bike i've put a 4 into 1 on generally performed better across the range, not a huge amount, but enough to notice....... As a matter of interest, I had a GPX {Ninja} 750 race bike and put it on the street, It had a huge amount shaved of the crank, when the previous owner had it done, everyone said he was mad, he would lose tons of torque, that was one of the fastest street bikes i've ever ridden and it was an old bike {1988}, it blew the doors of much more powerful bikes and easily kept up with the previous owners GSXR1100, it was a production racer as well {one of the fastest in the country at the time}, not a superbike, I saw over 150MPH on that bike on the street, it had tons of usable torque as well as revs...
I've only ever owned 3 bikes under 750cc, a R5350 {given to me}, a cb450 {given to me as well} and a 1983 GSX550es, the smaller capacity bikes had to be revved harder to produce both torque and peak power anyway and its considerably further up the rev range than the bigger bikes, its the nature of a smaller engine. As I said earlier, you won't get a true "performance" 4 into 2, so just put any 4 into 2 on it or get a set of 4 into 2 headers and make your own mufflers, like 2 cone engineering mufflers, it would both look and sound cool, it is a street bike after all and it sounds like you aren't that young, so who cares if it isn't a true performance pipe....?
