6'4'' and currently **cough** 360 pounds myself (I cough because I was down to 305 not so long ago

) and I have to agree that its very likely a stretched chain though a RD adjustment can also be the culprit (I have had both issues on my K2 mountain bike) I would try adjusting the RD and if that does not work then look into the chain.
You mentioned upgrading to X7, does the bike have SRAM components (shifters/RD) on it now? if its Shimano (and I am not 100% on this but am pretty sure) you may need to stick with a Shimano RD if you have Shimano shifters/RD now.
on the Wahoo, I rode one over the summer (ended up buying the Xcal) and it should be fine.. though don't hold me to that