i have a 90 and already have 1 40. i guess 2 would be an overkill eh... although the turbulent flow you could get would be beautiful.

Just keep them turned down a bit. You can get some pretty sweet waves goin if you have two of them. Or you could always pick up the MP20 then upgrade it at a later date.

With 2 of them running at max velocity yes it would be.

That would be 6000 gph or a 67 times turn over rate in the tank.
I know several sps guys running their systems at a 50X turn over rate though.

In anti synch mode though they will be on opposite cycles though so then you will have excellent flow in the tank that is also alternating.

We have a local client here running 2 MP40's on a 90g corner bow and he just loves them. We were able to create a 1" wave in the tank even with the 2 pumps being at 90 degrees to each other. As the pumps do not need to be turned up very high they were also relatively quiet in this mode. And in lagoon mode he runs then in anti synch and they are almost silent!

It depends on what your keeping in your tank, SPS 2 would be good (not at full blast), LPS not so much. I have a 4' 120g and i am running 2 40w @ 90% on lagoon mode as well as 3 korallia's (SPS dominated tank) i had to shelter my bubble and my frog spawn though.

Would the mp20 suffice for a 24" x 24" x 18" sps dominated cube?

I thought it should be fine, but to be certain, I verified with Vortech and they also thought an MP20 should work fine for your tank noirsphynx.

Think 1 MP20 would make a wave in a 45g corner Bow?

On a 45g bowfront I think you might be exceeding what one MP 20 can do. The wave dynamics in a bowfront are a bit trickier.

I know on a 34g square tank one MP 20 will work very well, as you can see in this link.
Click here to visit our site and see an MP 20 running on a 34g Solana

The nice side of an MP 20 on that size of a tank is that the one pump gives you 44 times turn over rate in the tank :)