I can see two issues: "overpowered" is probably less of an issue than the reinforcement behind that mount. Read up on it, and I think you'll find the conventional wisdom is that your stock Catalina bracket is going to be undersprung for the heavier motor.....(which probably isn't really that I of a thing.....you just have to help it more)......there's several ways to deal with that.
You'll probably need to go check out how well the mount itself is backed up on the transom. Some kinda G-10 backing plate, or metal angle, is probably in order.
As for the bushings, most of the people I have read have stripped the mount apart and gone to a good local hardware store to source some replacements on the cheap.
My boat didn't come with mount at all.....I just purchased a salt water approved "Fulton" mount.....with heavier springs.....because I intend to buy a new Tohatsu 6hp Sail-Pro when I get the boat in the water.
If it were mine, if look up how much the motor weighs, and source a mount that is designed to deal with its weight. Not because it'll be easier to lift, but because the stock mount looks kinda flimsy to me.....especially with that burly motor.
On the plus side: I don't think you'll have ANY issue with power! You could probably sell the 15, get a new little motor of your liking, and have cash in your pocket.....and a new mount!
(The Merc 15 is an awesome motor......in my area, that would bring $$$ from the driftboat guys!)