I'm in the "nothing ventured, nothing gained" crowd. Although not a Tormach, my mill is about the same size. Two years ago, I replaced the spindle motor with a 3KW (4HP), 2 pole motor. Mind you, all my work is in aluminum. The original motor was suppose to be rated at 2HP, which I really doubted. Naturally, I had folks telling me I'm nuts. I heard things about spindle dropping, bearings failing, finish quality, don't use a 2 pole motor, etc, etc, etc.
Anyway after close to 2 1/2 years, I can say that this has been one of the best things I ever did on my mill. My finishes have actually improved, my cut times have been reduced. Again, I only work with aluminum, thus the 2 pole motor worked for me. I've already run parts with 10K spindle speeds and the spindle purred like a kitten.
Here is an older video of the mill facing a piece of stock, using a 4" face mill....
The motor is a Binder 2 pole pump motor. I bought it on Ebay for $100.00. I had the rotor re-balanced at a motor rewind shop that I use to work for. I'm using a Hitachi WJ200-022SF drive with this motor. I initially had a Huanyang VFD (which worked great), but, decided on the vector drive, to basically learn how to use them
One thing I did notice, when I made my first cuts. The machine had a more solid sound when cutting. I'm sure this is due to the weight of the motor, adding some mass to the head.
Anyway, enjoy your adventure....it's stuff like this that make new ideas happen