"Anyone think it is possible to day trade stocks (not futures and close out positions by end of day) with buying power in excess of $20 million?"
The following is certainly reasonable:
(note that each block of 20k could be replaced with (4) 5k blocks - making 20 trades or 40 round trip.)
20,000 shares GOOG @ $285 = $5,700,000
20,000 shares SHLD @ $155 = $3,100,000
20,000 shares UTX @ $107 = $2,140,000
20,000 shares CME @ $230 = $2,300,000
20,000 shares TOL @ $96 = $1,920,000
Total: $15,160,000
One can certainly expand on this.
Now, would one want to trade in this fashion? I would certainly find it more useful to build positions in a variety of stocks over a certain amount of time.