That would be the North Power XT extension.
I've never tried one, so I can"t give you an opinion.
Ith should work to allow you to crank the downhaul a bit more, but
I've always just stopped at the beach and used my Chinook Downhaul
Crank to change the downhaul setting.
You need to reset the outhaul if you change the downhaul, but with the
adj. outhual setup, you can just slck it off, adjust the donhaul, and reset the outhaul.
I'm surprised the stock 70 cm didn't plane up faster than the Mongo.
Maybe look for a little more powerful profile in a 70 cm fin.
Hope this helps,