Mugen was founded by the son of Soichiro Honda. probably as close to OEM quality as you will find. Spoon is just a tuning company.
Why not boost? A GSR responds tremendously well to boost, especially turbocharging. Kits are easy to find. You get a lot of bang for your buck.
Besides, if you want to outrun turbos, you will have to spend a lot of money. Is it really worth it to be NA? Also, there might be a few turbo GSR's out there, but how many are hacks? To find a well tuned turbo car is just as rare as a well tuned all motor car. Once you have had a taste of boost you'll agree. The rush of endorphins throughout your brain are released when you feel yourself pushed into your seat as boost is rising, then the VTEC engages and it is like getting kicked in the back. Boost is a wonderful thing.

2000 ITR turbo.