I run the OS.52fs on a LT40 trainer and it's a great engine.
Ultra reliable and nice running, but not too powerful. It does an excellent job on the trainer and is good for basic manouvers, but I do not belive it's a good 3D/Hover engine for a 40 sized 3D plane. For general sportflying, sure. For 3D, maybe not.
I have also two Sig Somethin' Extra's now.
One have OS70fs and the other one has OS52fs. I have only flown the one with the .70 so far and it's crazy much power IMO

SSE + OS70fs goes vertical and you can really feel that there's power to spare (I'm no 3D flyer though so I haven't tried any hovering or such.)
If you're interested in 3D type flying and have a choice between the .52 & .70, then go for the .70.
Also, for some reason, the OS70 is cheaper than the OS52 here in Europe... Check it out!