Sorry to be slow with this reply - just back from the ASN in Chicago ... but in my view, you look like you are sitting pretty nicely as you are.
That said, I am a believer in low blood flow, gentle dialysis if one doesn't need to go faster and while 275 isn't a huge pump speed, we tend to recommend between 225 and 250 ... especially on our 5-6 nights per week home nocturnal patients. To be honest, while we once had a few 7 night/week patients, none now do 7 and most do 5. But, if you are well, able, and committed, then I would do whatever you feel comfy with. As for turning down you pump speed and any effect that may have on your B2M clearance, I don't think this would make a meaningful difference, as B2M clearance is largely duration related. certainly you will lower your membrane pass rate, but not greatly. I think the potential saving of your fistula may outweigh any deficit that may result in B2M removal.
In my view - and Andreas Pierratos would agree with me on this ... and he said exactly this last week in Chicago ... there is no evidence whatever to suggest an over-dialysis syndrome - perhaps with the single exception of the potential for increased AVF infection. That said, our experience has been that the longer in-vessel dwell time our patients experience through long hour, frequent duration dialysis has not been to a higher infection rate.
The risks of AVF infection, again in my view, all come back to meticulous care of the fistula, early recognition and action if/when an AVF looks at all inflammed, and meticulous needle stablisation. And, in our view, no-one looks after an AVF better than the patient him/herself. In my view, the somewhat higher rate of AVF infection in the nocturnal arm of FHN2 trial may have been more a signal of poor needle training and AVF care rather than needle dwell time of needling frequency.