I am currently using a pair of Swift Audobon 8.5x 44 which are great --BUT-- I would like a pair of Leica or Swarovski ,is the difference worth spending another £600?
When I first started birding I used a pair of Swift 8.5x44 Audobons and they were excellent optically - I have since upgraded to a pair of expensive roofs - but it was partly prompted by my Audobons needing cleaning after getting them wet - i`m not sure the difference is worth £600 but you do get to a point where the peace of mind of waterproofing + a little increase in performance seems worth the extra money!!
Optically i noticed mored difference when I upgraded my scope - so if you`ve money to invest it might be worth looking at that first.
However an expensive pair of bins is a pleasure to own and handle - if you`re using them regularly it might be worth it on that basis alone.
Out of interest why would you like a pair of Leica or Swarovski? Are you after the best optics or want something flash (not a criticism btw!!) if the later then any of the top names will do, if the former you'll have to go along and try them and make the highly subjective decision on what is worth what.
The thing to bear in mind that in optics like anything else there is a dimishing rate of returns. In good light you'll notice little difference. Its only in poor light that the differences shows up.
It doesn't matter what anyone else says to you or however scientific you want to be bins are highly personal and the descision highly subjective.
Have a look at the big names on a bad light day and see what you think.
Oh if you're not into looks then have a look at the Nikon SuperiorE 8x32 everyone who uses the things raves about 'em.