Just depends on the particular rod. Calling a spey an "8wt" doesn't really say much about it as far as its grain window. My guess is that there are just things on your cast that might need work. You can learn to cast just about any head on it. If you're using a 600gr, you're probably already toward the high end of the grain window.
For perspective: I've been fishing a loop yellowline 12'4" 8wt and use a skagit switch 540 + tips from 12' T-11 (672gr.) to 12' of T-17 (744gr.) Other friends of mine who fish the same rod use 570 compact skagits + 15' T-14 (780gr.) while others use 630 compact skagits and 10' of T-14 (770gr.). All 8 weights are different but I hope that gives you an idea of the various combinations a given rod can handle.
Without knowing what rod, 600 gr seems to be in the ballpark on most 8 weight spey rods. Your most likely having a break down in your casting sequence. For that if your skagit casting I would recommend the DVD Skagit Master #1. One quicker possible solution is you might be pulling your anchor. Try using a longer tippet, somewhere around 4+ feet. If you increase up to a 700gr line you will most likely really have to slow down your casting stroke. (again depending on what rod you are using)
I would say go for it if you want to jump up to 700 grain it can't hurt I watched skagit master 2 a few day ago scott throws a 750grain skagit with heavy sinktips with the 13'4'' z-axis 8wt
for example on my 13' 7/8 tfo deer creek I can throw 570 grains compact skagit and 15feet of t17 make that whole set up 825 grains and the suggested window on the rod is 450-700grain but is all personal preference I like to load the rod on the heavy side and chuck a lot of junk
I run a 600 compact skagit on my 8129 z-axis and couldn't imagine going heavier or lighter. Before the 600, I tried a 570 and just didn't like it nearly as much. Although this is a rod that some say just wants more and more. I just haven't felt the need to go heavier, but have to say I am intrigued. Regardless, a perfect example of casting stroke and personal preference being most important with regard to your set up.