I did a couple of tests to compare 24 bits exports with 16 bits exports. It seems that with 16 bits exports dithering noise is added but without a specific noise shape. To make a long story short: it sounds very bad.
I am a fan of iZoptope Ozone3. The MBIT+ dither noise it uses (all noise shapes sound great for this type by the way, but Ultra is the one I love the most) is really great sounding.
But even Type1 or Type2 are much better than the dither noise Reason is using since it sounds like quantizing noise; harsh and digital.
For a future request I would love to see some nice sounding noise shapes in Reason, or at least Type 1 or Type 2 for those who want to export to 16 bit in Reason. I personally find Type1 and 2 a little noisier than the iZotope MBIT+, but that's maybe a very personal thing.
That's it for now P-heads