I'm going to do a little resurrection over here.
INS basicly has no nerfs, and solid buffs. [...]
After playing some games with the strat this was also my feeling. The only "nerf" that you can feel is less range for the transports, even then -1 is not that impactful.
INS feels as strong as IF, defensible like PD, cheap like GW without its downsides.
I don't think changing the transport cost will do the trick, even if we go +50 or +80. Insurrection has too much money for it too matter in the longer/larger games.
1. I would try this: reduce the transport capacity by 20%. So Naval 15 -> 12, Air 5 -> 4.
2. Alternatively It might also be worth to consider nerfing movement a bit more. Maybe -2 for Naval and -2 or -3 for Air Transports.
3. Both? If one is not enough.
INS plays to me like a zombie infestation. You snowball like crazy, when fighting neutrals, many times ending with more units after the battle than when you started. If you manage to somehow takes someone's city with 0 units you can take over like cancer spreading through their back lines.
The problem is that you are too mobile while being strong, cheap, easy to mass, hard to dislodge, with a potential to turn 3 units into 7 and then 7 into 14 and 14 into 32 if you find a weakness in someone's defenses.