The main benefit of studs is that they hold the torque better than stock head bolts, and don't seem to stretch like stock bolts do. Over time, stock head bolts will eventually loose their grip on even an NA engine, so if you keep a 6.9 long enough, you'll be doing head gaskets someday. If you put studs in when you do them, then you will probably never do a headgasket again. The ability of the bolt or stud to keep the clamping force up enough to prevent the gasket from blowing out under high boost seems to be the main factor in why we can't run more boost. Actually, it's not so much the boost, but the actual cylinder presure that presents the problem obviously, but we tend to talk in terms of the boost being the culprit. Water injection is supposed to lower the internal cylinder presures. How, I'm unsure. But it is said you can run higher boost with water injection then without. What that magic number is I'm not sure, but I'm pretty sure that on a 6.9, you'll blow a gasket in the high teens without water.