Wow.. so i had an hour to myself today and figured i'd try out the hop up rubber "Dental Floss" mod as found on this youtube video
Hopup perfect seal mod DIY (floss mod) Airsoft - YouTube
I did everything as shown in the video and you can see from the pictures attached there was a remarkable increase in my FPS!! I was expecting in the ball park of +10FPS but a jump in approx +30FPS is insane. I guess i didn't have a very good air seal to begin with maybe.. I'm sure the differences will vary amongst different guns.
Definitely worth looking into though