OK, so I just setup the 3.5WM's on the dash, wrapped with a towel, so that I could work on determining a decent position for a pillar install. I'll say up front that the Utopia M's in a two-way (6WM's+TBM's) do sound really good. The 6WM does a nice job of conveying the midrange and meeting up with the tweeter. Once I added the midrange though, the 6WM became a midbass monster! Since it's only covering 70-500hz in the three-way config, it just really lets loose. For some of the songs I was testing with, I just turned the sub all the way down. I'm sure I'll need to tone down that midbass, once I get the mids installed and start tweaking with it. For anyone thinking about the 6WM, it's pretty strong as a midbass.
I did notice that having the 3.5WM's did fill things in a little more and brought up the soundstage a bit - more solidity to instruments and vocals - and a bit more detail. Overall, once I get them installed and do some tweaking, I think I'll be really happy that I added the 3.5WM's.
Thanks for all of your inputs,
Dave