Personally, I have not had many tuning issues with my GRP .21 . It has an awsome engine. Mine is going on its 3-4th gallon? and still runs great. I did, however, switch from the GRP #6 (TOO COLD) to the GRP #4, which smoothed out the idle, got rid of the occasional flame out (after refueling). I am now running the OS p-4 plug, and seems to work great. It runs all day @ 180-190 degrees, with tons of power. As far as holding a tune, I find that it normally only needs a tiny tweak of the LSN (within 1-2 hrs) depending on track, but normally is fine.
I have read of some having tuning issues, but I have not experienced any of that, with the exception of the ones I mention. Maybe I have been lucky.
I come from running the Novarossi's (which I still like), but have been SUPER happy with the GRP, so I see another in my future.
Oh yeah, and I run the Jammin JP-3 on it, which it seems to love. On race day, I have gone 8 1/5 minutes with fuel to spare.
Just my experience, anyways.