I first heard one of the Op. 44 tracks from this CD on my local classical radio station, Classical New England, and indeed it was the first time I had heard any of the Op. 44 pieces, period. I enjoyed it so much, I borrowed the Pacifica CDs from my local library. I compared the Op. 80 performance with one I own by the Merel Quartet on the Genuin label (http://www.amazon.com/String-Quartets-Felix-Fanny-Mendelssohn/dp/B004TNZVMQ/ref=sr_1_2?ie=UTF8&qid=1385308368&sr=8-2&keywords=mendelssohn+merel), and judged the Merel's performance and recording acoustics superior. This led to me to explore other artists who had recorded the complete set of Mendelssohn's string quartets.
There's been a lot of buzz about the Pacifica, and they continue to get quite a bit of exposure on Classical New England, but I ultimately settled on buying the Henschel Quartet on the Arte Nova label (http://www.amazon.com/Comp-Str-Qts-F-Mendelssohn/dp/B0009DC9CU/ref=sr_1_21?ie=UTF8&qid=1385309339&sr=8-21&keywords=henschel+mendelssohn). I find Henschel's performances terrifically nuanced - they're not just plowing through the material; they're inhabiting it and fully bringing it to life. Also, the acoustics of the Henschel's performances are just about right - the recording venue for chamber music is critical and needs to provide a certain amount of resonance (technically, this may not be the right term, but I'm referring to the sound quality that represents the difference between a large performance hall vs. a recording studio with sound-absorbing, baffled wall coverings). While I found the Pacifica to be technically competent, the recordings are dry - they lack sufficient acoustic resonance for my ear to find pleasing. On the other hand, the Aurora Quartet on the Naxos label (e.g., http://www.amazon.com/Mendelssohn-String-Quartets-Vol-2-Felix/dp/B0000013ZG/ref=sr_1_fkmr2_1?ie=UTF8&qid=1385309806&sr=8-1-fkmr2&keywords=mendelssohn+aurora+complete), which is also highly rated, sounds like it was recorded in a performance hall that was a tad oversized - too much acoustic resonance.
I have sampled every other ensemble that ever recorded the Mendelssohn string quartets (quite a time investment!). The Melos Quartett on the DG label (http://www.amazon.com/Mendelssohn-Streichquartette-Complete-String-Quartets/dp/B00000E33G/ref=sr_1_1?ie=UTF8&qid=1385308001&sr=8-1&keywords=mendelssohn+melos) was a close second for me, but I found them to be shrill at times. For me, the Henschel Quartet represents the high water mark for the Mendelssohn string quartets, where this amazing music meets its match in terms of sheer performance talent and engineering quality.