I've read on some sites that it's possible, and even fairly simple, to upgrade the stock turbo on the Octavia Vrs II with a little bit of tweaking by a professional mechanic, as an alternative to fitting an entirely new turbo. Any thoughts on this regarding plausabilty/safety/performance/fuel demands, etc?

Not heard of it but I believe that you can uprate the internals to increase power but I'm really not sure. Stu will no doubt be able to shed some light on this.

Turbo's spin at many thousands of RPM, so I would think any changing of internals would require specific balancing and be a case of taking an old one and sending it off to have the parts changed over that the whole assembly balanced.

I wouldn't want a mechanic messing around with the internals of a turbo unless they were a turbo specialist and had access to a lot of specialist equipment.

Not heard of anything myself.

The guys in the US are using bigger turbos but obviously the ECU needs to mapped to make use of it.

My advice would be not to fanny around with the original turbo, just save the pennies, be patient, and wait for the release of Revo Stage 3, which will use a Garrett GT3076R turbo with an external wastegate. Nice.