Interesting topic...here's some thoughts from a guy who has had a half dozen turbocharged street bikes and put almost 50,000 miles on the last one in 4 years.(Honda Blackbird (intercooled)with 255 HP and 155 lbs tq. on pump gas)
Head porting: This is probably the thing to do last on a big horsepower build. Generally, the combustion chamber is smoothed and polished, a larger exhaust valve is fitted along with heavier springs, and the port is enlarged. It's expensive, not many guys do it right, and it yields minimal results. My 1216 Bandit with MC Express (intercooled) blow thru kit had the head done because we were using 20 lbs of boost...(287HP and 187 lbs torque on race gas)
Valve springs: Only use as much pressure as you need to get the job done. With turbos, the old rule of thumb is add 1 lb of pressure per lb of boost.On low boost applications, up to say 10 lbs or so, springs are generally not needed unless the motor has marginal valve springs to start with. Suzuki Bandits and GSXR's are an example as they had pretty low seat pressure to start..
Turbo vrs NA built motor..I had 2 200 HP Bandits at the same time. One had a 1340 motor that Bob Carpenter built. It made 195 HP and 109 lbs tq on race gas. The other Bandit had a Mr Turbo draw thru turbo and made 190 HP and 110 lbs tq. with water/methanol injection. The turbo was much easier to live with on the street. I could idle it in traffic, drive it normally, got good mileage and the power was only limited by boost level. The NA motor was highly strung and while I never had any problems, was more of a time bomb. It required way more maintenance, didn't like traffic, and didn't like to be lugged down. Between the 2, I generally jumped on the turbo.
With turbo bikes, you want more horsepower, you just turn up the boost.
I generally ran 10 lbs max with pump gas, 15 lbs with av gas, and used race gas for high teens.
Hope that helps you guys out a bit...Hank
2012 ZX-14r
2000 Suzuki Bandit Intercooled Turbo, 287HP, 182 lbs tq...sold
1997 Honda Blackbird Intercooled Turbo, 255HP, 155 lbs tq...sold