A 10×31 CWP basically raises all gear ratios by about 10% thus giving it “longer legs”. I fitted mine because, at the time, the Légère was my every-day car and I was regularly commuting between Berkshire and Yorkshire.
The prime advantage is that it will enable you to cruise all day, very happily, at top motorway speeds. I found it made little real difference to either top speed, fuel consumption or acceleration. However, having raised all the gear ratios, it did effect town driving noticeably in that, at 25/30mph top gear was on the high side, necessitating a lot more gear changing between 2nd and 3rd.
Similarly, on long or steep inclines it was likely to run out of puff earlier than one would like so I became very adept at dropping into 1st gear on the move. That said, it held its own when the TOC did its Norway rally to the Arctic Circle – those “hills” tried it to the limit but it coped admirably, albeit often in a lower gear than the other cars.
Bear in one my experience of a 10×31 is limited to a small bodied car and you are apparently considering it for a Big 15. I do know of one fitted to a Familale and, despite the engine being in good condition, it struggled on steep hills due to the extra weight. That owner eventually reverted to a 9×31 as a result. In honesty, I don’t think I would consider it for a large bodied Traction but it probably comes down to how you intend to use the car.
I was thinking about this over the last few days driving our Normale from Sussex to and around the Lake District. I would agree with Bernie. A higher ratio would have meant more time in second going up and down the hills – as well as all the dealing with all the roundabouts and traffic lights on the way -and we wouldn’t have got anywhere any quicker. I’d leave it as standard.