You can get the Orbe clamp at a reasonable price from UK online retailers like Analogue Seduction (if you're not in the UK already.) It's a simple upgrade that I wouldn't hesitate to do first.
http://www.analogueseduction.net/product/Michell_Engineering_Orbe_Record_Clamp_Upgrade_Kit_For_GyroDec_and_GyroDec_SE_ME-OCKI've bought several Michell upgrades and parts from them.
With this upgrade you keep the Gyro platter and replace only the spindle and clamp.
The tonearm de-coupling kit is also cheap and effective.
You also get the Orbe clamp if you upgrade to the Orbe platter, but not everyone seems to like the effect of the fatter platter (well at least one guy I know of who complains that it has negative effect on PRaT).
After that, the HR power supply upgrade (PSU) is rather expensive. I got mine on ebay for about $500. You could also get this from UK if you don't mind modding for the voltage difference yourself
http://forum.audiogon.com/cgi-bin/fr.pl?eanlg&1168816683By the way, the stock power supply does have a speed adjustment, but you have to open it up to get to it.
I think the Audio Note/Rega arm is a good base from which to explore other cartridges. (I don't know anything about Sumiko cartridges.)
A new tonearm would probably be the most expensive upgrade and could cost more than the table itself.