Sleeving has been done for many years in the UK, with lots of guns being sleeved. As I'm sure you know, guns are re-proofed after sleeving. It is a good way of extending the useful life of a good quality gun without the extreme expense of new barrels.
One thing to bear in mind is to be sure that the new barrels (and additional weight thereof) doesn't upset the balance and handling. Many best grade guns were made new with walls that were thin by modern practice, especially towards the muzzles. Sleeving tends to increase the weight 'forward' as modern barrels carry more metal there. To extend the length as well may push this to a gun that is very 'forward heavy'.
Generally, one of the attractions of 'best' guns is the wonderful handling, but even quite small changes can upset this on carefully set up guns. Adding weight in the stock may recover the balance, but it doesn't recover the 'handling' as the whole gun has gained weight.
