I think the guy on the website is confused.
Normally when talking faster/slower, they mean acceleration, and your speed from a stop is slower when you lower the rear sprocket, but you get a higher top end (and normally better gas milage if you keep your rpm's lower at the same speeds).
But since your now using more hp than before you have less reserve for things like wind as he noticed (aka wasted gas). Only solution you wil have is to downshift, if it is a problem.
select yamaha, xv, 250 88-09
It uses all the same gearing except comes with a 45 rear instead of 42rear, I have no idea about chain though, as the 2010 manual doesn't spec the chain at all.
I would likely if I was doing it, go with 16/38 max, 17/38 is likely too much. and it means you won't have to replace that sprocket also, the front ones can be a real pain to get off.
Also check what is currently on the bike, not sure if you got the bike used, someone might have made changes already.