Hi and welcome to the Cult of Shovelglove!
First things first: what's RSI? My mom is a nurse and my dad is a massage therapist, so I'm not used to seeing medical acronyms that I don't understand.
As for the sledge, if I were in your spot I'd probably upgrade to a 10#, though I'm not in the habit of optimizing my workouts for safety so I don't know how valuable my advice should be . . . Depending on the life of your exchange policy, you might want to go on playing with it for a week or so before making up your mind. After all, one workout is just a drop in the bucket when you're doing it 5 days in a row, and you might find that the 8# will catch up with you quicker than you expect.
I'm guessing you already know this, but in the mean time you can try to get more out of what you've got if you hold the sledge further from the head to give you more torque, do more reps/set or just go faster. All kind of obvious I guess, but without knowing more about how you've setup your routine it's all I got for you.
I wouldn't worry too much about "wasting time", because even a light workout is still beneficial (how beneficial depends on your goals and your level) and for me there's always a "break-in period" with new workouts where I have to play around and do them at less-than-peak performance while I figure out exactly how to make it as challenging as I can. Aside from that, there's still the habit-forming aspect of just getting up and doing it everyday when you're supposed to, regardless of how heavy the weight is.
I hope my semi-coherent rambling helped, but if not I'm sure one of the more experienced shuggers will chime in with better advice.
Until then, keep on shuggin'.