I have never heard of dry sanding gelcoat but if it works for you then fine. I just wouldn't recommend it to first timers buying an old boat looking to rid themselves of oxidation. If this was a forum for professional automotive or gelcoat restorers then I'd say you are good to post what you say, but somebody that has no clue and goes this route can seriously F it up in a few minutes. I value your opinion as well as the others on here that have done it themselves. There has to be a level of comfort in this for a weekend warrior in knowing he will eventually reach his goal of oxidation free gelcoat and not "Oh my God, WTF have I just done?"
There is definitely risk vs. reward going 400 dry. I hope you understand my point.