Question is in the title! Seems like to buy an SLX-GS mech you need to pay about £5-10 extra. Can you buy shorter mech plates and retrofit them? I already have an SGS mech…

Probably supply and demand.

Would suggest that 95% or more of bikes that are sold have 3 chainrings, and an 11-32 or 11-34 cassette, thus necessitating an SGS or "long cage" mech. So not many GS or "short cage" mechs get sold, hence higher prices.

It's often the same if you want cranks of any length except 175mm. If you want 170's for instance, you'll usually pay £10 more in my experience!

I've been running GS with 3 chainrings and 11-34t for years now? Works fine on a hardtail if you don't use daft gears such as 44-32.

GS is medium cage, SS is short cage, which is more or less single chainring issue i think.

Pretty sure you can order most shimano parts as spares, the madison site certainly lists lots of spare parts, and the shimano site gives part numbers for pretty much everything. Wonder how long you'd have to wait though…

Get a GS, when it wears out transfer plates onto your next cheaper SGS mech. assuming you didn't trash the plates 🙂
I'll bet the plates seperatley cost more than the extra £s you spend on GS

http://www.bikeradar.com/gear/category/components/derailler-mech-rear/mountain/product/rd-4500-8918

The RRP on both GS and SGS is exactly the same, £59.99

You can buy the replacement cage plates, they are £9.99 and £5.99 so sixteen quid to convert a mech.