I have a 2007 forester with 200k which developed a head gasket leak.  I rebuilt the motor having the heads and block both resurfaced.

Prior to rebuilding the engine, the transmission was surprisingly strong.  In reverse and on a hill, it would almost surprise you with how fast it would back up with just a little throttle.

After rebuilding the engine and reinstalling,  the transmission seems sluggish.  Drives forward with little effort but shifts a little harder than it did.  Reverse is where the issue is.  Same hill mentioned above, it will hardly back up.

I have checked fluid and level is fine.  Transmission does not appear to be slipping and fluid does not smell burnt.

Any suggestions or help on what to check would be greatly appreciated.

Make sure the Hill Holder is fully released, if it has it.

Dent in the trans pan from lifting?  It only takes a little.

Of course, anything mechanical needs to be addressed, however "Lucas" transmission treatment is quite good.  Check the fluid, etc..  I did a change and filter replacement last year.
 

Was the torque converter properly seated before the engine was refitted? 

Cheers 

Bennie