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It will depend on the boat weight, mounting height and prop you select.
One could get 30 mph if mounted all the way down with an oversized aluminum prop or close to 50 mph.
While not the expert on that hull, I don't think an XL shaft is appropriate and will be too tall. You will need to mount it on a 5" or more jackplate.
I hope Finnegan will give you the definitive answer since he has several classic Outrage models, runs Mercury engines exclusively and is very well versed on performance tuning.
The classic 21's use a 20" engine for both single and twin configurations. For twins, the engines must be mounted up 5 bolt holes (or four holes up if that is all your engine brand allows). I would not recommend a 25 " engine on the boat at all. It looks awkward and requires a jackplate to get it up high enough.
