I've squeezed a 12" into my Champ XD. I used a Celestion Century vintage neo magnet speaker - overall weight of the combo only 2lb heavier.
I have (brutally) sawn off the metal edges of the speaker, leaving all the cone area intact but only a sliver of the surround is attached for a few millimeters at the top & bottom. The max height availble in the combo is 11". So far this has been no problem, it has stuck fast even on the loudest bass notes.
Currently I am firing the speaker through the 10" hole, which seems to sound OK but makes the treble very directional, and undoubtably will affect bass efficiency also.
Next step is to make a new baffle with a proper 12" hole! I will recover with Fender-like cloth & screw on the logo to make it look stock.
Results - I would say at least 6dB louder - equivalent to a power increase to 60W. Tonally slightly different, a bit brighter possibly but there is extra low from the 12" speaker balancing this out nicely. Have gigged it with a band once & had no probs with output level.