First, don't even think about going "much above factory". That is the very reason so many companies produce loading manuals. Full power factory ammo for the .357 is loaded to pressures up to 36,000 pounds per square inch and that is the industry maximum. They have set that standard so that experimenters like you don't garner nicknames like "Stumpy".
Compare the two guns and you will see that the larger chambers of the .45 leaves far less metal for chamber walls therefore requiring lower overall pressure limits over the .357. The Uberti is fully proofed for standard factory level loadings, whether .357 or .45 Colt. No, the .45 Uberti will not withstand loadings listed for "Ruger Blackhawk or T/C ONLY" but will handle loadings of a 250gr bullet up near 1000fps, and that is nothing to sneer at. With your preferred Thunderer/birdshead grip, I believe that you will have found about all of the fun that you can stand. In reality, this load will do about all that could be expected of a handgun of this size at reasonable ranges, and that is certainly well beyond throwing spitwads. Take care and BOL.