I think the 85 dB receivers should work fine with your hearing loss, but I don’t see much wrong with upsizing to the 100 dB receivers if you want to have some margin for future proofing in case your hearing loss gets worse than what the 85 dB receivers can handle later on.
I’ve heard some argument that using an oversize receiver may increase the harmonic distortion, but honestly I’ve reviewed the harmonic distortion specs for these receiver sizes and it’s so low that even if there’s a slight increase, it’d be minimally unnoticeable by you. But last I checked, they have the same percentage of harmonic distortion rating.
Also, I think if you drive the 85 dB receivers very hard near its edge of performance, my personal opinion is that they’ll probably be failing sooner rather than later. But with 100 dB receivers for your 85 dB loss, there’s some room there for margin so that the 100 dB receivers don’t have to be worked right up to their edge of performance all the times. This may help make them last a little bit longer.
The obvious drawback I can see is that if you have small ear canals, it may be harder to fit the bigger 100 dB receivers into smaller ear canals than the 85 dB ones. And even if it still fits, it may feel a little less comfortable if there’s not much room left for the dome to seal up your ear canal and the whole fitting may be too snug, more than necessary.