to answer your actual question...i have a 6.8 ribcraft with a 250 df on the back.
the boat came proped more for load ie: 6 adults so has a 19.5 inch pitch prop.
currently at wot (6100) she'll do 47 knots....i could (should) reprop imo and get a 23 inch prop which i would think cut the wot but increase top speed slightly when light loaded
as to the engine on this hull...it is very stern heavy and drags at slow speed so much so i have toyed with the idea of trim tabs as well as another prop but money stops this just now.
the boat takes the 250 fine (it was fitted by ribcraft) but if i were too re engine the hull after having this one for 4 years...i'd be looking at a smaller engine which hopefully would be a bit lighter...at a guess i reckon a 200 would be right...
your offer of a 175 i could see you'd be pushing it hard more than if you had a 200-250
i can run the 250 at 4000rpm and be doing 30-35 knots which with the family aboard is plenty.
as to customer service i went in last month and had nothing but help and great advice from jason behind the counter.
ps as a little thought....i've just had to replace my trim ram with a new one as i had a pea size hole corroded through the oil gallery..
I must have a bad earth somewhere on the boat ut have now got a bonding wire which connects the ram to the anode which wasnt on it when i bought it.