First, the Physx card is not necessary with TRI-SLI, I have done it and yes it will help your Vantage score, but unless you are playing a lot of games that take advantage of it I would skip it.
Second, I do run TRI-SLI with 3 x GTX280's, EK full MB block and the Heatkiller block all off the T3 and no it did not hurt my temps. However, since you already have a loop setup and are happy with it, price for a MCP355 and XSPC res top would be around the same, so I would go with what you are comfortable with. I use a triple Thermochill PA120.3 rad x 2 in my setup. I would suggest a triple rad or at least a dual Feser or dual XSPC RX series rad for your GPU's, skip Physx and go with the Heatkiller blocks with the TRI-SLI connector placing your cards in the first three x16 slots. It will also allow you access to the reset, power buttons, USB connections, etc. all the stuff on the side of the board. It looks like you have room in the bottom of the case for a dual rad, not sure where else you can do it without going external.
Make sure you have at least a 1k PSU, mine have hit as high as 910 watts under load, probably a little higher, but that's what I was able to catch.
Here is a PIC of my current setup:

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