Single flute carbide cutters

[Jonathan]

4 flute end mill doesn't leave much space for chips at all, so they're going to get re-cut / rub etc causing the poor finish you describe. 0.3 DOC at 350 mm/min is pretty pathetic ... as Jazz said you should be able to get a lot more than that.
On my router, which compared to a mill isn't very rigid, I've been using 1.2mm DOC (sometimes 2mm for short periods) at 700mm/min and 12000rpm. Clear the slot of chips and put some cutting fluid in it before the cutter gets round.
Since your max spindle speed is 2700rpm, not 12000rpm, you want to reduce the feedrate roughly proportionately for the single flute cutter to get the same chip load and increase the depth of cut.

The video I posted earlier in the thread should give a reasonable idea of what it looks/sounds like. Finish is generally good - here's an example where I didn't do a finishing pass as such. 23mm thick if I recall correctly:


And another with a different grade of aluminium (showing how grade can make a big difference):