I've heard the the REVLOC is fine, but has one problem. I have NOT used it however, so these comments are 2nd hand. As the rpm's increase it gets harder and harder to pull in the clutch lever if you decide to keep one.
I don't know anything about the EFM, so I'll skip that. I decided to go with the Z-start. Reasons were 1)-cost, 2)-pressure limiting on so that the clutch has an even pull-presure at all rpm's, 3) -easy to install and very adjustable in rpm engagement range and for what rpm it starts to engage. Here is a link that I hope works. It is a clip from an XR650 autoclutch thread on Thumpertalk that really goes into the Z-start. By the way, because Z-start hasn't tooled up yet for the XR650R, I am still waiting for mine, so just keep in mind-I haven't used it yet, so I can't say whether the above is true or not.-david
http://www.thumpertalk.com/bike/ubb...391278&page=0&view=collapsed&sb=5&o=7&fpart=1
If this link does not work, e-mail me and I'll send some photo's of the Z-start to anyone who wants