Regen field is one of if not the least used upgrades in the game, and I think that's a shame since it is unique and adds something new to the game. The real problem with it is it takes up the same slot as gun, is almost the same mass cost as gun, but is much worse than gun. Here are some changes I think would improve the viability of the upgrade greatly:
Increase range to 30, same as gun range. This would allow a much larger portion of your army to receive the buff.
Give a 1.1x health boost, same as Advanced regen field. This means Thaams survive 1 more ACU hit with 8 hp, and would help a great deal with facing off against a gun com.
Remove ACU HP boost. The 1.1x HP boost is a pretty significant buff, and I don't thing the ACU HP boost makes sense anyway. Sera already has T2 and Nano regen as health boost options, there really isn't a need for a third.
(At least) double power cost. Regen field is currently 600 mass 4,500 power, compared to 800 mass 24,000 power for gun. ACU upgrades tend to be power intensive, and regen field being 500 e short of being buildable just from base storage is a little odd to say the least.
Let me know what you think!