Is it acceptable to decrease the HP rating of a blank-face GFCI?
Are you sure that it's no powering more than those two recepticles?

Yes, you can downgrade it from 1.5 to 1.  You SHOULD however, make sure the new GFCI you want to install is rated for 20 Amps.

As for why they chose a dead front, most likely it was used for a spa at some point and local code forbid use for anything else.

It could also have been that the owner intended to reserve its capacity for some other high current device.

Some deadfronts have multiple load terminals which could possibly be a reason.  (To avoid wire nuts).

Or perhaps it's a 240v GFCI.  Nobody generally has use for 240v 20A outlets.