The "logistical headache" of re-positioning BA aircraft to be based out of MAN is actually not that hard. VS already carries this out with their ATL service in which aircraft swap routes out in ATL therefore allowing major maintenance to be carried out at LHR without the need of a positioning flight. BA could easily do this with the frequency of flights they have to multiple destinations they have in the USA. The 787-8 is the perfect aircraft for these types of flights as it was what is was designed for long thin routes.
With regards to diluting the market DL added their JFK service last year and yields on AA have roughly stayed the same. BA could possibly make the 2nd service somewhat later in the day as they have done for their new LGW-JFK service.
Other flights I would like to see from MAN would be:
MAN - WAW (LOT)
MAN - DXB (Using QF Metal)
MAN - NRT/HND (JAL/ANA)
MAN - BKK (TG)
MAN - DEL/BOM (AI/9W)