I have a 430/21 mast and a 48cm extension and this works good on a 6.6 sail.

I decided to buy a 50cm Extender (like a 3rd piece of mast) extending the mast to a theoretical 528 cm so i could get a big sail on it, around 8-9m, without buying a new mast.
I had concerns over the stiffness of this mast in such a big sail
(say something like a Neil Pryde V8 8.4m, luff 506cm, suggested mast: 490/29)so I asked the guy in the shop when i bought the extension and he said it would be softer than specified but work ok.

When I went into another shop to buy a sail the guy wouldnt even entertain the idea of test-rigging on this mast combination: it'll be too soft.

I presumed that as the wind i would be using this in would be v.light then getting everything perfect would be less critical.

Similar size sails (8.5 ish) such as North S-Type state a mast of 490/29 or 460/25 with extension, so is it too much to push to IMCS of 21 ?

If the mast isn't right the sail just won't rig properly.
Sometimes the battens won't rotate properly.
This sometimes happens even if the IMCS is as specified but other mast bend curve characteristics are not compatible.
Best to use the recommended mast if possible or at least the recommended length and IMCS