G0MJW Mike, changing the LO to 9.5 GHz results in a higher IF, lying more into the spec band width of the chip. Couldn't that be the 1.6 dB MER difference ?
No I don't think so as I also found other things that worked, even just a few Hz from 25 MHz made a difference, which makes me suspect it is related the Leo Bodnar reference division ratios. I also tried 24 MHz but that was not very good. The PLL multiple is 390 for 9.75 GHz (424 for 10.6 GHz) so if you wanted a 430MHz If you would want about 25.79MHz. I tried 23.59 MHz which puts the transponder on 23cms but it wasn't very good. Best for me seems to be 9.5 GHz.
Other LNBs may be and probably are different.