I get by with my 2KW dewalt router and can cut aluminium ok with speed set at 8-10,000rpm with the right cutters. I think it depends on the rigidity of your machine. If the spindle has decent bearings and little run out you should notice a good improvement and be able to use much smaller cutters. I can get down to 0.5mm dia with my dewalt but after half a dozen uses the cutter often breaks. I will be going to upgrade to an Elte high frequency spindle when my dewalt has had enough, these have minimum runout, ER25 collet (so a much larger range of cutters will be available without using reduction sleeves) and ceramic bearings, much less noise decent price (compared to the likes of Perske and Columbo) and variable speed, thing to remember with a spindle is if its rated say 18000rpm 400hZ, reducing the speed will reduce its torque.