Mine is the Mkll, not M5G, but I also have and love the Herbies mat. As Wayner suggests, have a think about the mat height first though. I have added a 1.5mm plain rubber mat under mine as I have the 3.7mm Herbies and the extra rubber simply added the extra height to centre my VTA adjustment for my range of cartridges. Your set up needs may be different. A thicker Herbies would have done the trick for me if I'd thought it through first.
As to the rest, I agree with dialoum. The standard arm is great (rewired in my case, not needed with the M5G) with the KAB damper and I also swear by a Jico SAS in a Shure cart. These upgrades to me were huge for a moderate cost and I no longer have any desire to change the arm.
I also fitted a Mike New bearing early on and this was also a big improvement and thoroughly recommended for refining detail and reduced noise.
It is relatively expensive however - to me worth every cent as a long term investment but perhaps not to someone on a smaller budget. Finally, I found that fitting an external power supply gave a further worthwhile improvement but to me it was not as big as I expected so down a bit on the value for money stakes IMHO.
I've done nothing with the plinth or platter and have simply screwed the original feet up tight as I have the luxury of a solid wood encased brick platform underneath so easy to level manually and gives minimal feedback.
