I can reiterate that the mod made my mu sound better - it's plenty quiet for mastering, and I've been using only Manley-supplied tubes, so far, since I performed the mod, myself... I also personally performed the HPF mod, and have loved the sound of that.
You may have seen everyone's favorite EE, DC, suggest the JAN 5-stars for lowest noise. But, low microphonics is another trait that one can shop for, if that is your concern. Still, the Manley stock I've used have had neither symptom to any objectionable degree (touch wood). And the quality of the compression became prettier, somehow, after the mod. I can use more threshold than before and not feel that I'm burying the transients. I often flip the HPF, too.
I thought I could do away with the mu after installing the OCL-2, but they are different types of tube compressors and answer different needs. The OCL-2 is so versatile that it actually handles 95% of my tube compression needs, but for some stuff - such as raunchy rock, the odd song, or just "bad digital" - the mu can have a palliative effect that none of my trafo-less gear can touch. The OCL-2 is perhaps overly truthful to the source to do that kind of analog alchemy.
Per EveAnna's advice, I turn on the mu at the beginning of the session and let it warm up (at least long enough to make Lava) before using it... also, per her advice, I turn it off if I'm sure it won't be used for over 3 hours. It gets turned on for mastering at least 5 days a week. The tubes seem to last and last (again, touch wood).
Andrew