If you want to make a bike lighter then you will need to look at all the components. But the bike has to perform well also, so sometimes it is better not to push to the limit. We have an interesting choice when it comes to the shifters: either I choose very good performance or I can choose the lowest in weight...
Thumb shifters in a Quest are unusual, but I have used Shimano Ultegra thumb shifters in my Mango Sport and I'm very pleased with the accurucy and ease of shifting. The steer in Mango and Quest are the same, so I see no reason why it wouldn't work.
It is not particularly light at 194gramm, but still some 20gramm lighter than the standard X9 gripshifters that come with the Quest. I could have bought the 900TT, but it's only 16 gramm lighter and costs some 40 euro more. Most probably both working equally well, so it's not worth the extra bucks.
This is a cheaper shifter at 63 euro incl. shipping (found it even cheaper later on at http://luckynino.blogspot.com, tip from Jeroen Koeleman). At the moment it is the only production 10-speed gripshift and it was only by accident that I saw them on the site of the lightweight cassette manufacturer RECON. The SEC S-light 10-speed shifter comes in a version for SRAM XX derailers and a slightly cheaper version for Shimano derailers. Their weight is minimal and also much lighter than the SRAM gripshifters. We had a Shimano type SEC shifter installed on a bike at work. My first impression was that it shifts precise enough, but very very light and not consistent in all the gears. When I made a testride, I sometimes upshifted unwillingly two gears at a time. Maybe it's something to get used to, we'll see.
The low weight is tempting, so I will try these first. If I find these shifters to be ok, I might sell the 500TT shifters.
More components will follow later.