Or how much weight this conversion adds on top of the stock 6sp? I am considering getting a 8 or 11 speed IGH conversion but am not sure if I want to take the extra weight hit.

We have a Brompton S 6sp with springy brooks and a stock Nova. The Nova feels significantly lighters. When I weighed the two, the difference was only 3LB which caused me to realize that every pound counts even if it is only 5-10% of the bike's weight.