I don't know about power user but I have a c128d/1581 jiffydos and 512kb expansion. I also have a naff cartridge switcher, that should allow "switching" between carts, but certainly the retroreplay doesn't work with other carts plugged into it.
It's all quite new kit for me, so I haven't used it a great deal yet - apart from installing a case fan some heatsinks, and replacing the vic (it had a faulty one which didn't work after it got warm. my previous fix was a large heatsink, + cpu fan! - in the end I got a replacement)
