Hi Jim,
If you paint or powder coat the heatsink area then you will be in good shape! That should correct any emissivity issues the metal heatsink may have.
Regarding the (TOBJ - TDIE) range - that's a general-case condition. With calibration, we've been able to get accurate results even when the temperature delta is much higher.
Let me know once you have some final data gathered and I'll be happy to help you with a full calibration.
Best regards,
Ian Williams