Don't get too excited about the under suit.
I have a MTM Santi under suit and it hasn't made any difference. My dry suit is an O3 Lycra and probably the most flexible and stretchy dry suit available.
I am starting to think the only thing thats going to make it a safe option for my diving is a front zip on my dry suit. At the moment I can reach all my valves and do shutdowns but its so painful and slow it would be useless in an real emergency situation. I wouldn't trust it as an emergency procedure until I could do it quickly and with ease. Screw this up in real life and it could kill you.
The stiff zip across my shoulders has to almost bend the wrong way to let my arms move back that extra 50mm so i am hopeing thats the prime culprit. The only thing I have found that makes it doable easily at the moment is a wet suit
ATB
Mark