An SL2PLL is designed for an offset dish with FD .5 - .6. Like any other KU LNBF designed for this higher FD, only a portion of the 6' dish will be illuminated.They work, but only will receive a portion of the reflected signal.
The GEOSATpro SL1P, Invacom ADF-120 with LNB or the unit in your photo is designed for prime focus dishes and should illuminate the reflector properly. Of the three, I personally would use the Invacom ADF-120 and the Inverto Black LNB. The Invacom ADF-120's FD is adjustable where the SL1P and the one in your photo are fixed.