If you are willing to reconsider the pin and collar type, these alloy extensions from RADS work really well for me and are relatively inexpensive.
http://www.sunshinecoastsailboards.com.au/shop/components/sail-components/mast-extensions/radz/radz-rdm-euro-pin-aluminium-detailEuro pin system
Two pulleys that run well
A really good cleat that works well with downhaul cranks
Easy to adjust and simple reliable pin system
Allows collar to be removed for very short extension if needed.
I have never seen the need for carbon extensions. The weight is right down low where you don't feel it and I think alloy is a bit more durable/reliable in this situation.
Note the threading has no crossovers or extra friction with right angle pulley orientation.