The "extra semi-tone" on the "full Boehms" is there solely for transposition purposes, much as is the low Eb on the bass clarinet (a 1900's addition to enable bass clarinet players to transpose the infrequent (but still present) A bass clarinet parts).
Most players will not use it at all. I will occasionally use it on the Bb horn for a middle line Bb to C trill, but other than that, never.
In turn, having a low Eb on a A clarinet is of even more limited utility for transposition purposes (unless there is a Ab "mezzo soprano" ancestor hiding out there somewhere).
In both cases, this extra tone hole and key does have the wonderful effect of evening out the B natural in the middle of the staff. Even on the best of R13s, this is a note that sounds "different" than the others to the player (if not to the audience).
For the other fingerings, I find that I use the left hand Eb all of the time, followed next by the articulated G# (which is a godsend if you spend a lot of time in extreme sharp or flat keys, as I do on many of the vocal numbers that I have to play) and then by the forked Eb/Bb. (Over the last five years or so, I have started to use it a bit more - again, mostly when playing parts in musicals.)
As for extra weight - well, to someone who plays the bass clarinet off of the thumb rest alone at times (much easier than wearing one of those two hook abominations of a neck strap, if you ask me), soprano clarinets of any style are feather weights. Your experience may differ of course.
I've said it before and I'll say it again - I would do everything short of killing to maintain my access to the full Boehm clarinet. It duplicates what I have on the bass clarinet, as well as one of the more reasonable workarounds (the articulated G#) on the saxophone, it puts the problem G3# tonehole on top of the horn where it never collects any condensation, and you never have to "slide" or fake it with F#/C# to anything else fingerings again. A lot of problems, all gone with very little effort.
Had I not obtained a Series 10 with all of the bells and whistles save the low Eb last year, I'd have been in the hunt for the current matched pair. (For the record, I still prefer the Series 9 bore to that of the "R13 imitator" Series 10.)
I only run into Eb clarinet once in a blue moon (usually during Bernstein shows these days), so I couldn't make much of a case going beyond a thousand or so on the Eb. But, I will be interested in what the pair goes for, particularly due to the reasonable price.
Regarding Leblanc horns, I think that they still list the Pete Fountain horn as an available instrument for regular purchase. It comes with the articulated G# and the fork Eb (but not with the LH Eb lever, to my knowledge). 'Twas a time when you could special order any combination of the various "extras" from Kenosha, but not any more.
(And, I'd advise against Amati. I bought an Amati Oehler horn (top one that they offer) three years or so ago, but the fit and finish was not up to your typical student horn, much less a "professional" instrument. It took quite a bit of tweaking (and the construction of some custom "thinner" cork tone hole pads to cover the vent holes operated by the brille) to get it all set up right. I'd not wish that ordeal on anyone.)
(Note to self: make plans to sabotage Oboe Boy's internet connection during future auctions...)
