Yes - HSS does cut AL better than carbide. It also seems to have less harmonics than carbide because it is not as dense. When I need to do thicker AL projects, I use HSS end mills from OSG. I believe the helix angle is more aggressive on the HSS cutters for AL to get the chips up & out faster than a standard end mill grind.
HSD RPM - It depends on how low your VFD is programmed. The V1000 is programmed to run 5,000 RPM as the lowest limit. In reality, you won't have enough torque to do any meaningful work below 8,000. The motors are wound to be routers...not milling machines - so you may have to increase move speed to get the right chipload to match the higher RPM...or deal with light chiploads.