Five days later, so hope this helps...
I pretty much never see anyone talking about MidCap Indexes as even a part of their allocations, but personally I'm BIG fan, and VIMAX is my core fund. It correlates very closely with the total market and is actually mostly a Large Cap fund of companies/brands everyone recognizes (Large Cap funds are really Mega and LargeCap).
As you've seen, VIMAX consistently does better vs. Total or 500 Index over nearly any timeframe with fees almost as low as those two. So, if the 500 Index is a reasonable exchange for the Total Market (which a ton of people here agree with), I think VIMAX with ~350 companies can be, too.
But if you split between the MidCap and 500 Index, you'd have ~850 companies (I think there's no company overlap) that are mostly all Large Caps, but less emphasis on Megas and more on Mids than the Total Market alone.
Back-tested over decades, it's done much better than the Total Market during accumulation, and FAR, FAR better during draw-down. Plus it let's you rebalance without the need for bonds.
I don't find there's much of a diff in taxes, but I also flip VIMAX back and forth from Vanguard's MidCap Growth and Value Funds (the two halves of VIMAX). I also flip between the 500 Index and Total Market.
Similar-ish annual results with either combo, but by doing that I keep resetting my Cost Basis so there'll be almost nothing to tax when I retire.