Re: 6hp Tohatsu for Irwin 25
I had a similar weight Catalina 25 with a 9.9 Tohatsu. When it became time to service the thing I always wished it was a Tohatsu Sailpro 6. The 40lb weight difference is significant.
In pushing against a strong headwind you might use the upper ends of the power on the 9.9, but in most conditions you won't be needing it.
Catalina's recommendation is for an 8hp motor. Most people go for the 9.8/9.9 because it's the same weight as the 8hp and not much more expensive.
boats that I sail on regularly: Pearson 28-2, Rondar 505, Yankee 30, Blanchard Junior