what is the max ammount of boost the 6.5 can handle or the GM8 can put out?

In stock form I'd recommend no more than 10 psi sustained boost with occasional peaks above that, this is with gages without gages don't diddle with boost or fuel delivery. With WMI or IC to post turbo cool then 13 sustained is safely possible with higher peaks, some claim it's okay to run higher, I choose not to run that high sustained for longevity concerns, 10 psi is plenty of boost, not much you can't do with that much boost.

GM-8 can go to 22 psi but above 17-18 psi turbo starts getting inefficient in terms of energy required & backpressure to spin it vs gain of power resulting from higher boost

I've seen my gm-4 put out 21 psi once but I've also seen a cracked piston, too.

I'm surprised none of you guys on here have reported a GM-X grenade going off under the hood. 6-8 psi stock, jump to 15+ psi is in the neighborhood of triple the speed the turbo is spinning at. I'm getting ready to pull mine off soon for a rebuild, compressor wheel is worn, and the bearings have a little play...man its nice to work in a diesel shop ;)

I just replaced my center cartridge. The rear bearing was the one that had failed the most and groved the turbine shaft and eaten the exhaust wheel 10 * or more. I didn't hear it make a sound, I just happen to check it and the slop was there.
I've installed one of those rebulid kits on a center cartridge too. $75 from J&H turbo in Ms.

QM,

You got a phone # to J&H? or do they have a web site? Id like to rebuild my GM-8 and need a fairly complete kit. I noticed a few pits on my blades and some slop. What all comes with a center cartridge?