Well as you can see in post #6, 06-08 wrx 4eats came with a 3.9 ratio so I don't think it would be that much slower, plus you'd probably get a little better gas mileage. Anyways I was just wondering in case I ever decided to mod the engine then I'd also want to improve the power train and the stock rear diff seems to be the weak link.What differential do you currently have? You should have a R160 in there, which should hold up just fine. A 3.9FDR with the auto will be pretty slow, since I think most 4EATs come with a 4.444 to offset the torque converter's sluggishness.
The bigger issue is the transmission conversion. Is it 1:1 or 1:1.1? That makes a difference on what ratio the front differential will have. I can't remember which transmissions came with the 1:1, as I think most Subaru transmissions are 1:1.1.
It is far easier to swap the style of differential (open vs. VLS, etc.) than changing the drive ratio of the differentials. If you are going to put a R180 back there, you might as well swap the 6MT it comes with into place. I don't think any 4EAT or 5EAT came with a differential that big. Don't quote me on that, as I gave up trying to remember that information a long time ago.