I really don't care for the mutants and masterminds system as it's basically a cludge of a level-based system where there are many excellent systems already out there for super-powered campaigns that work fantastically.
Um, have you ever played M&M? Because I've played M&M, Hero System, and BESM, and I can tell you that all three, with their points-based, effects-based systems, play similarly. The three have their strong points and weak points, and are thus better suited to certain types of games.
Mutants and Masterminds - Remember reading comics and seeing the freewheeling battles where Hulk throws a tank at someone, or where the X-men are in a free-for-all against a Sentinel? That's the kind of feel M&M has. Everything is more abstracted, making it good for most superhero games. Good for a wide range of characters, though some types may be a little hard to put together (it is something of a nightmare trying to put together a person who can drag someone into the digital world where your Computer skill determines your abilities, best to stat it up like a kind of dimensional movement).
BESM - The version I played was with the tristat system. As the name (Big Eyes Small Mouth) suggests, it is best for anime-style games. You can do someone like Inuyasha or Kenshin easily in this system, but someone like Iceman or Jean Grey would be very difficult, since the system isn't really able to handle that kind of range.
Hero System - Of the three, Hero System actually plays more like D&D than M&M, which is a d20 system. Rigidly enforced tactical movement and hit points, along with other consumables, make this system almost like a superhero simulator. The flip side is, of course, the metric ton of work you have to do. Want a challenge? Try making a Hero System character without a calculator, spreadsheet, or character generator. It is... not pretty.
Of the three, I prefer Hero System for a long-running game, since if I put the effort into making a character, I want it to be worth it. M&M, though, is simpler, easier to use, and I can throw together a character in under an hour. For many players, the fact that they already know 80% of the mechanics (and the ones they don't know are close enough) you have a much lower barrier to entry.
For me the main issue for adapting D&D d20 system to shadowrun is the huge scale of power between fresh and experienced characters.
This is something I already do not like in fantasy settings (i.e. a lvl 10 group cannot be bothered by town militia, which limits the adventures that can be played). But in SR setting it is a show-stopper to me.
If this point can be solved one way or another, then I guess an adaptation can be acceptable, even if obviously not perfect
I'd suggest looking at M&M. PL 8 would be 'street-level heroes' (Daredevil, for instance). Your cops/soldiers/mooks are typically PL 6 or so. The fact that damage is based on a Toughness save with increasing wound penalties (like Shadowrun), makes it so that even a bunch of mooks can seriously wound and/or kill a PL 10 character. One issue, though, is that M&M in general has a much lower lethality rate than Shadowrun does. It was designed with superheroes in mind, after all. But if you were looking for somewhere to make a conversion from Shadowrun to d20, M&M would be the place I'd start, along with d20 Future.