Your mic is self-powered, so stepping up to 9v bias from the recorders "plug-in power" won't benefit you at all (and may not even be good for that mic). IMO, the mic is not bad but it's still on the entry level side of things, so it's not like the MD's pre is a weak link preventing you from realizing the mic's sound quality. Unless, either the mic is overloading the pres or they do not provide adequate gain. I doubt either is the case with that mic and a typical band, but let us know if it is.
Assuming the mic output is within the useable range of the internal pres, I think you'll only get a small improvement from switching to the SP (haven't heard the other unit) and I personally don't think the benefit would outweigh the expense and greater inconvenience of an extra box and an extra battery to keep charged. That's a judgment call, so others could differ. I do own a HiMD and an SP pre (until someone buys it).
A higher end pre that could make a bigger sonic improvement and also continue to serve you well in a higher end rig would be another story, but you'd be in for money.
Yes, should you choose to get either of those pre's, you will still record in stereo. Not all mic preamps are stereo (or more properly, have two channels).