So card arrived this morning, and I've already gotten it installed. As previously stated, I had updated my Nvidia Web Drivers to the latest (378.05.05.05f02 at the time of this post), so today I just powered down, removed the GTX 745, put in the p400, and powered on and it was just working without issue.
Just from normal use (i.e., stuff like YouTube), it seems great. Very smooth, seems issue free. Cinebench R15 scores are pretty identical to the GTX 745 (OpenGL at 63.98 fps, CPU at 876 cb), so I'm not sure if I can say for certain if it's an upgrade over the 745, but definitely on par. I'm going to use this for a few days while I wait on a P1000 to be delivered, and then I'll send the P400 back. I imagine the P1000 will be an upgrade over the GTX 745, and should be the best GPU I could possibly get for my build (based on my power and space requirements). I also like that this P400 has mDP 1.4 instead of HDMI/DVI-D (P400 has 3, P600 and P1000 have 4), and they come with mDP to DP adapters, though I will just use mDP to DP cables for my 2 monitors. All in all, very happy!
I probably won't do power draw tests with the P400, but will once I get the P1000.
GTX 745 on top, Quadro P400 on the bottom.
Quadro P400 installed.