Hi
Since I bought my '06 258 Vista it's not been achieving either top speed or intended max WOT rpms. I always wondered why. Over this winter I decided to find out. It has the 5.7 GXi-G 320hp engine with DP-SM drives that FW seems to favour. It revbed to approx 4400 with top speed of 32 knots on a good day, 31 the norm. It ought to do 37, so at least 400 rpms short and 5-6 knots.
Last year I replaced the external fuel filter, it was rated at max 50L/hr which is too little. Replaced with Racor 200L/hr 10microns. No difference in performance

and less in my pocket.
First thing this year was to get rid of the filtermesh on the fuel regulator. Found some black rubber stuff on there but not enough to reduce flow. During this procedure I had to remove the flame arrestor...lo and behold...it was full of what looks like dust (as in you know dust bunnies from home). Found the ultimate way to clean out is pressure washing (water) from inside to push it out.
Haven't had a chance to try out what it meant until yesterday. With full vacation packing, twp adults, two kids, water and fuel tanks brimmed we easily reached 34 knots and 4800-4900 rpms before I easied off. Also noticeably quicker to get there. The loss of top speed is easier to identify as those measurements are usually all empty, smooth bottoms with no paint and in saltwater, 34 iand climbing is a great speed given the circumstances. I'm sure empty, no packing, factory spec batteries (I carry 2 extra) i would probably get 1-2 knots more, and that's in fresh water.
So conclusion - nothing was wrong, just eight years of dust buildup in the flame arrestor. I was ready to invest in a new motor...thinking the 450 hours it has on it was what one could expect.

Mike