Material to machine?
Largest diameter end mill will you need to run above 8k?
What you are talking about is getting the added feed rate that higher RPMs can deliver if:
Vibration is minimal
Tool Tip extension is not too long from your current gage line
And since you have a couple of CNC machines, some adapability (from 1 machine to the other)
In My Opionion, the NSK electric auxilary spindle is the way to go... (for 1/4" max shank)
http://www.nskamericacorp.com/prod_machineTool_hes.aspx
If your need is common enough,
then you'll want to be able to rebuild the spindle (not as easy on the mechanical type)
Have two, so you won't be down if one is in repair
The mechanical ones that rely on planetary gears that increase speed up to 7x (Not to exceed 20k) and the extremely crappy air turbine type are a waste of money.
The extremely smooth Programmable Variable RPM (you can't hardly tell it's turning) and the short extension from gage line make the NSK auxilary spindles hard to beat. These guys live in high speed RPM ranges up to 60k...
I highly advise the necessay steps to prohibit a tool change when one of these are in the spindle (perhaps get someone to wire in an off switch for the tool changer).
Personally tested many different types, and this is by far the best route.