First off, its all-in-all a very cool idea :) BUT ^^' i think those modifiers are a "little bit" too strong. My suggestion for the modifiers are:

I don't think growth time will be positively touched by this mechanical world, but build time may be.

-40% Habitat ability; that there is no atmosphere is not a problem because of all the structures on that world, which could be airtight. BUT organic lifeforms do like some nature and since habitat ability is just another name for max happiness it should be displayed this way. And since all mechanical lifeforms get +200% habitat ability, they still don't care…

-10% happiness, because of the absence of nature; robots and droids still don't care

I don't know if -100% Food/Mineral hits it right since you still should be able to artificially generate food on this planet, but much more cost intensive. And since those buildings have to be protected from the vacuum building maintenance should go up in general.
Is there a way to set building maintenance +50% or so instead?
It also would simulate that a structure of this planetary size eats much energy, or just put up build cost.

+ on physics and engineering OK, but society… meh, difficult. Maybe because only Synth can actually research and it would be interesting with which kind of society they would come up as undying individuals, which in turn would be interesting to study ^^ but that's all very theoretical… in the end all these +250% modifiers are a bit overkill and since you can build multiple of these planets I would say +25% would still do the tech boost ^^'

+40% energy production. See that way you would counter at least the energy production penalty of robots and still have a boost for all future iterations of mechanical lifeforms.

I think this way you still get powerful energy productions or think tanks out of those planets, without breaking the game THAT much ;P

…planet builds with mechanical lifeforms only are already approximately 10% more effective than with organic lifeforms. They need energy only, which is a global multipurpose resource, not local and single purpose like food… with this planet type they get boosted into the endless ^^

...hope this helps :)