Rikki Tikki Traveller said:
The only way to make it work would be a 2D table that listed Performance on one axis and TL on the other, with %Hull as the contents.
Them's big tables.
I see two distinct issues here.
One is improvements, which is what RTT and I tabled out above. I don't see any good reason to have finely detailed rules for improvements. But I think working through these rules is jumping the gun slightly. Not that I don't like to see ideas posted!
The real issue I think is differentiation by milieu, and by empire, and by race. The terms are generally interchangeable in this context. And I think part of the solution comes from TNE and T20.
MGT's drive tables represent drives produced and shipped everywhere in the Imperium, and perhaps neighboring empires. Common, and excellent.
So I think differentiation should come in
(1) via Quality (which RTT could suggest as an add-on, right Hunter?, or Marc could suggest as well). This attribute or set of attributes could be retro-applied to all equipment and vehicles as well, assuming it can be mechanically integrated into MGT;
(2) via "milestones" by which drives are incrementally bulkier or svelter. But I think they're best implemented with multipliers, even as simple as [base TL / current TL ] * base volume -- though I maybe prefer only two "early" stages and only two "late" stages, generically applicable and relative to the "baseline" TL for whatever technology you're working with.
(3) via "new" technology. Power plants are a big example of this: Fission through TL8, Fusion from TL9, and Antimatter after that, and who-knows-what after that. M-drives are less easy to replace, but I bet they could be. Force ships to use HEPlaR until TL11. Or not. Then phase out the M-drive around TL18. Replace it with Stutterwarp (just kidding, but you know what I mean). J-drives ... well, they're J-drives. I don't care if they ever change.
All three of these working in tandem could create a rich design environment without requiring huge tables or hard math. It would frame your design efforts by milieu, and guide your decision-making process too.
...of course, there's always the game-mechanic offset solution as well: if your ship is of an inferior TL, you get a -DM. If it's superior, you get a +DM. Simple and effective.