The reason why many people (incl. myself) like the MP2 as a preamp for the MBHOs is this: It has "warm" characteristics which means it slightly over-emphasizes the lower frequencies and tames the high end a bit. The MBHO KA200N cardioid caps sound rather bright and have rolled-off low end, so the mics and the preamp work together nicely.
I assume the stock UA5 would sound a little too neutral/transparent for the MBHOs. The warm mod would be the way to go if you want to keep the UA5, I guess, but I've never used that combo, so take this with a grain of salt. You could also get an MP2 and use the UA5 as A/D converter. Not sure if it's worth carrying the extra box though. Let your ears decide.
In any case, the MP2 is just a really good box for field recording, and you won't regret getting one in the long run. It's relatively small, built like a tank, easy to use, and it runs on AAs.
I use the MBHO cards and the MP2 into and Edirol R09, and I love the combo. Great sounding, very rugged, and stealthy (if required).