Absolutely. No question. I've got three different types of D/A (Apogee, Lynx, and Dangerous Audio) and there is a marked difference between them all.
When I first got them we (a colleague and I) did a shootout between them and an RME fireface, a Presonus Firestudio and the Yamaha converters in the 01V96. The high end converters definitely sounded more detailed and just more exciting to listen to. Transients were more defined and stereo imaging got better. Switching back to the prosumer stuff, the playback material just seemed to slump.
We're in audiophile territory here, which I'm not really mad about, but I can definitely say that the playback quality was improved as a result of higher quality D/A. And I should hope so. They cost a fortune.
But yeah, the first thing you should be worrying about is your room and your monitors. If those aren't up to scratch it matters very little what D/A you have.
Oh, and for the record, yes, my mixes improved as a result of having a good D/A. But it was one of many factors that contributed in the long run. Proper gain staging and not overdriving the analogue section of a cheap D/A, for one, will make more of an impact.
Cheers