One query - is this a Jones underlever or a push forward or thumb hole "underlever"? If a push forward, which I have on my 1870 Reilly hammergun, it is incredibly easy to use and I notice virtually no difference to using a top lever and that is on either clays or live quarry. Although the Jones underlever can take slightly longer, with practice, even this is wholly managable.

A (UK) sleeving job of £450 strikes me as too cheap by far, especially if one wants a quality job done.