This has always been the case in my experience with past betas. I think they may use a different server possibly for them regarding Siri. Once the official release comes around the exact opposite happens and the performance and reliability degrades.
Ah. Havent tried it with Hey Siri. But dictation seems to be better now. I do seem to recall that being the case with the last beta, too. And like shockwave said, it went back to being mostly crap after general release.