Poseidon will be coming out with a swivel turret in 1-2years, but they do not make sealed diaphragms and this next one will be similar to the MK3 on the top side.
Tek3 as mentioned is an "adjustment" of the DS4 and there isn't really a way to put a bottom port on it. You can use a 90* adapter if you want to. They really aren't great for doubles though and I think a lot of people are using them for CCR bottles which is where they are pretty useful instead of using manifold blocks.
@pinshootr I'm not aware of anyone using them for sidemount. I personally wouldn't choose them for sidemount since the lack of swivel is a kink hazard in an air sharing situation.
I highly doubt you will be able to tell a difference with the MK25 at the first stage level compared to your Dive Rite first stage. the second stages will make more of a difference since the XT1 will still flow more air than the tank valve