I havent cut alot of sheet metal but the stuff I have cut (galvanized a/c and electrical work type) I had to go under 5krpm's( I wanna say best ran about 3500rpm) I used 1/4" 6flute coated carbide.
Like I said I havent cut much and if I went over 5k it would burn the endmills, I also sandwhiched the sheet metal between 2 alum plates with .010 clearance on profile.
Personally I would rather cut 304 stainless however its more than likely due to inexperiance on sheet metal and I have cut lots of nickle alloys and stainless but gal. sheet metal has kicked my butt every time.
the machines I used were fadals and also a hand mill for stupid stuff from buddies with projects my handmill I believe has a 5hp motor on it in never bogs down.
wish I could help better.