My boss has a 1995 Buick LeSabre limited with the L27. He just bought it and asked me to drive it. This car was beyond all DOGGY. The thing can barely get out of its own way, full throttle is just about needed to pull out into intersections. I asked him about it, and he told me that it'* peppy and feels great, up until the engine reaches operating temperature, then it gets doggy like that. To me, it feels like the car is starting off in third gear. When I drove it, I didn't even feel it shift a gear until 40 MPH. He mentioned that it drops in/out of OD at highway speeds too for no reason. To add to this, he is only getting around 15 MPG out of it and it has a rough idle. The check engine light is NOT on. (and there aren't any stored codes.)

Clean the MAF, check for vac leaks to start.

I did check for vacuum leaks, there aren't any. I've also swapped out MAF sensors already. :(

Check the fuel pressure?

Not yet.

Check the fuel pressure when cold, then again after it dogs down.

Could also be a bad cat. Suggest the usual run it around at night and look to see if it'* glowing or the kick the cat while listening for a rattle.