Re: Cheaper engine performance techs.
Thinking about it, while it's a buff to missiles (since the player can now choose freely between the range/performance they would have had at that tech or new, shorter range/higher performance missiles), I think it buffs them in an interesting way. Previously it was almost always correct to go with maximum performance unless making a specifically long range missile, with the reduced missile fuel efficiency in the new version + this change I think there will be more room for tradeoffs between longer range, cruise-style missiles and high performance, short range "rocket" type missiles.
One of my complaints about missile vs beam balance was that the ranges were so different that they were essentially incomparable as weapons - if the enemy was shooting missiles at 100 million kilometers (not uncommon at even medium tech levels) then it didn't matter if you were twice as fast as they were - it would take hours to close to beam range, and all that mattered is if you could survive until their magazines were empty.
Now, if the choice is between missiles with a range of 50 mkm and 30,000 km/s speed, or missiles with a range of 5 mkm, 40,000 km/s speed, and 25% bigger warheads, there's more room for tactics. Possibly a sort of rock paper scissors where cruise missiles beat rockets (range) which beat beams (can overwhelm beam PD) which beat cruise missiles (beams PD shooting down the slower missiles), but also possibly more complex tactics, like trying to run down missile ships before they can unload their magazines. I look forward to seeing the results.