Milling with spindle speeders

I have a cnc mill (Topwell) with a 10 hp spindle, 40 taper, box ways and seems to be a very tight and smooth machine. BUT, I can count the spindle rotations at only 4000! What are the limitations with using a speeder? Are they only useful for engraving? Do they require lots of upkeep and maintenance? Could I expect to run a 1/4 endmill at 10-20,000 rpm in aluminum with a 5X speeder? What about milling steel?
Thanks in advance,
Barron