I'd get rid of the Aluminum Flesh line (or at least change its effects). If you want more hit points, there's already a line of evolves designed primarily for that (Synthetic Mesh's +Con). The actual effect of the change in HD size is hard to estimate, varies by level, and depends in part on metagame choices (namely, the very common house rules regarding rolling for hit points).
As previously mentioned, the DR-bypassers can also move in to the same apartment and make a sitcom that people will watch. Bypassing DR is a specific but not entirely uncommon situation, but bypassing a specific type of DR is fairly niche. It's why Mountain Hammer (bypass all DR) and those augment crystals (bypass a specific type of DR and give bonuses against the creatures that typically have it, all for cheap) work, as does the golf bag (non-magical or lightly enchanted weapons made of special materials are surprisingly cheap). Since evolves are pretty significant, you can't just throw them away on such situational options.
Evolve ideas. I'm mostly focusing on Mech for now.
- Fortification
- Senses (how are there no scanners in there?)
- Alternate attack modes. Bio has 4 (tentacles, ooze/engulf, swarm, pheromones). Mech has, technically, 3, but they're all part of the armbolt chain so I have trouble counting them as more than 1. Either way, that's not enough. Let me bleed battery acid or something. Laser eyes! Shoulder-mounted auto-turrets! Maybe fire off some AMP (Anti-Magic Pulse) grenades?
- Some options for built-in activating (er, spellmolding), maybe? Such as giving some System feats out for evolves, maybe even the Activate Module/Socket Access feats? Probably not too much of it, though, or you start stepping on the toes of the actual activator classes and losing the original Synthevolver focus.
- Utility options. Bio has quite a few, but Mech is almost entirely combat-focused.
It would also be nice if there were a couple of final evolves that didn't have any prerequisites. Heck, some stage 2/3 evolves that don't have prerequisites. Bio has several 2s, but nobody has any 3s.
To support multiclassing and PrCing, I'd suggest using an "evolver level" or something (similar to caster level) instead of your raw Synth' level to determine the effects of level-dependent evolves.
Edit: Stabilizer needs a buff, badly.