I decided to go the SuperMax route in the end and have just received the modified sprocket.




The weight of the modified item is 2.02Kg (4 lb 7 1/8oz) compared to 2.82Kg (6 lb 3 3/8oz) before modification.
The costs added up like this:
Shipping of 'spare' alloy pulley to me from a friendly TMer: £20
Shipping to Supermax: £36.43
Supermax Process: ($535) £347.51
Return shipping: ($70) £45.47
Total: £449.41 GBP
I'm looking forward to fitting it in a few weeks' time when some other issues have been taken care of!