Finally finished - it took longer than I anticipated because of other projects but I'm happy with the result.
I found that I could simply add 4x1mm spacers for the front aluminium brace and file down the plastic bumper rather than trimming the aluminium bit itself.
I'd mention again that I think Mr. Tamiya should have included adapted parts for us to use rather than making us trim existing ones.
I'm not sure of the benefits of the thinner chassis, it seems to serve no purpose as it in itself doesn't really affect the centre of mass but does expose a little more.
But I am convinced the belt swap allowing the motor to be moved inboard will be a big improvement, especially for those with short motor shafts as I have - now the mesh is much more confidence inspiring.
But the front belt will surely rub on the servo stay now... shame:
For the extra teeth and the belt swap alone I think this was worth it but I can't imagine huge improvements in lap times. On the other hand I enjoyed the excuse to do a ground up rebuild of my 511, it's not something I need to do often and it was a nice reminder of the privilege we have to own such a nice bit of kit.