How high can you go on voltage?

There is more than one limiting factor. The ones I know of lie in the ESC, motor, and gearing.

From the ESC's perspective, they're all rated to a max RPM which is a limit imposed by the processing speed of the ESC. Most are rated to 150 000 RPM for a two pole motor, but some can go up to 180 000 or 200 000, and some can't even go over 50 000 RPM. If you run a motor of more than 2 poles the max ESC RPM goes down. 4 pole would be 75 000 RPM, 8 pole about 30 000 RPM, etc. if it is rated to 150 000 for 2 poles.

From the motor's perspective, if it's a 2 poler your first worry becomes bearings. A high quality 2 pole motor should spin over 100k RPM before iron losses become a problem, but I don't think any motors are supplied with ceramic bearings capable of sustaining those speeds. I know of atleast one person that has run a Hacker C40-12T to over 90k RPM with ceramic bearings installed. Unfortunately the motor shaft eventually failed though.

If the motor is a 4 poler, iron losses become a problem at about 60 000 RPM. For an 8 poler, 30 000 RPM, etc.

And the problem that I'll be contending with when I try to run 8S - gearing. If you can't gear down far enough for the speeds you want to achieve, your system is going to run inefficiently which can cause overheating. From my investigating so far, 30 000 - 35 000 RPM seems to be a sweet spot for car conversions given gearing options that are (easily) attainable.