I modded one of my SCXDs to run 5881/6L6GBs, and it puts out 25 watts at 5 percent THD and 30 watts at 10 percent. It's not all that much louder. The difference is in the solid bass tones and overall crispness from the larger plates and beefed-up power supply.
I think you'd have to go to the full 40 watts/6L6GC kind of output stage and a 12-inch speaker before you heard a meaningful difference in loudness. Frankly, I'd much rather see Fender put the effort into layered effects, especially separating reverb/delay from tremolo/vibratone.