Given that the 27 is rated for a max 9hp, have you encountered any issues with your 15hp? (2 cycle or 4 cycle?) I'm using a 5hp (4 cycle) extra long shaft on an elevator bracket and it moves me in and out of harbor easy as pie. I'm in a lake, so no real issues with the lower HP. Inquiring minds want to know!
I sail in the ocean off the coast of southern California. From Mission Bay/San Diego the most popular cruising destinations are Catalina to the north and Ensenada to the south.... either trip means one leg will be up hill.. so a strong motor is paramount if you're on a schedule.
My boat's Tohatsu 2cycle 9.9 hp has a factory injector kit available that will raise the HP to 15. The motors are identical otherwise. I had the conversion installed 5 or 6 years ago, cost was around $125. The extra power gives me more confidence in a seaway... opposing waves, wind, tidal current, etc. The only issue is increased fuel consumption. Up 20%... and it was already pretty crappy, Upside is they will burn old fuel readily, I've never, ever had a problem with old or ethanol fuel in the 2 cycle. Problem is... you can't buy a new 2 cycle in Calif. anymore.
When I acquired the boat it had a 70's era Johnson Sailmaster 15hp. Those and the Evinrude's of the era had a low profile cowling that allowed easy clearance for tilt up storage. If that is important to you.... a 6hp motor will most likely have a small enough cowling to fit... but check first...
The main reason I'd go with at least 8hp or more is so you can find a motor equipped with remote control... On a Cat27, the motor is difficult to access and control from the cockpit...the lazzerette hatch has to be left open. Having shift and throttle controls in the helm/tiller area... motor rotation locked.... and steering exclusively with the rudder, is a huge convenience.... well worth the expense.
If your 5 hp gets you where you want to go.... don't change it. If you're gonna cruise long distance and may get in some seas... (that includes choppy conditions with lots of vessel traffic) and you feel the motor struggling... perhaps increased power with a high thrust prop may be in your future.