Remember that the Saturn is a VERY complex system (one of the reasons why it had a general lack of 3rd party support in the US). The best emulators available for Windows require at least a 1-2GHz processor to even try to run emulated Saturn games at full speed, and many of those emulators have large tracts of highly-optimized x86 Assembly to get that speed (note that Yabause is written almost entirely in highly-portable C).
If someone wanted to take an actual approach to trying to get a Saturn emulator running anywhere close to full-speed on the Dreamcast, you'd probably have to make a lot of concessions. For instance, VDP2 emulation would have to be drastically simplified and made in a way that many games either wouldn't work right or would look horrible. VDP1 emulation would be doable at a decent speed, by mapping VDP1 sprites onto PVR sprites on the Dreamcast (Yabause for the Dreamcast already does that). These types of changes would probably make almost all the 2D games (which really was the strong point of the Saturn, IMO) completely unplayable (since they rely heavily on VDP2).
Also, you'd probably have to write the whole thing from the ground up in highly optimized SuperH Assembly. There are a few parts of Yabause's Dreamcast port in SuperH assembly (such as the low-level CD interface and the (incomplete, unused currently) SH2 dynarec that I wrote -- although that more generates machine code than assembly, but that's really irrelevant). However, to have any chance at full-speed, you'd probably need to write EVERYTHING in highly optimized assembly, which there are very few people around in the homebrew community that have the skill to do so at this point, most likely. Then you'd need to have the massive amount of time to actually sit down and do that coding. Not something you're likely going to find anyone to do these days, unfortunately -- especially not on a homebrew project that they have no chance of ever getting paid for.
Even with all of this, I have my doubts as to whether you'd ever get full-speed, even on a few games. There's a limit to how much overclocking your system will help, because I doubt you'd be able to get a stable system running at more than 400MHz. That's still nowhere near the level that is required by emulators running on Windows.
Keep in mind that even after all this time, there isn't a full-speed emulator for the SNES or the Genesis on the Dreamcast that the homebrew community put out. Genesis Plus DC (with BlackAura's hardware renderer) came pretty close, and actually hit the mark on many games -- but made many of the same concessions that I've spoken to above in relation to the Saturn. Several games either will not work properly or will look so bad that they're unplayable with that type of rendering. DreamSNES also came pretty close on the SNES front, and with some tweaking was able to play some games at full-speed (especially PAL releases). The Playstation, which is vastly simpler to emulate for most cases than the Saturn (from my experience) has never had a homebrew community effort to even try to put forth a full-speed emulator. Bleem for Dreamcast is quite awesome (and of course did run full-speed), but then again the guys who wrote it were much better at this kind of stuff than pretty much anyone in the homebrew community today. Also, they did this for their full-time jobs, so that tends to help too.
I'd honestly love to see someone prove me wrong and put out a full-speed Saturn emulator for the Dreamcast. I just don't see it happening though. You're much more likely to see a full-speed Genesis or SNES emulator, as they should be doable with enough time and effort.
TL;DR: Even with overclocking, I wouldn't expect to see a full-speed Saturn emulator for the Dreamcast, period.