Max commons are okay. They pack decent punch.
The higher the level (which mean more power to next level), the higher the increase in the amount of power (for the kit). In higher levels you can get to 11K increase in power (per level), although you need to spend about 8K or 9K to boost that level (which might be discouraging)
Max epics are used on legendaries. A maxed epic will boost a (non premium) legendary item to level 33. A legendary that is below level 32 will not be maxed with the use of another epic kit (and will need some common food).
Legendary kits, in their name, should go with prenium legendary/mythed ones.
But again, those are pretty rare. You will not want to transform power kits.
You must also agree that by transforming, you need to spend gold and you lose some power.
Yes, I agree, having a power kit at 100K power (or 50K power) and use it to boost a dozen level on an item feels good, it might not be THE most economic choice.
Same as when you boost common kits to boost other items -- you can boost all the legendary with the commons directly (20% rule), but its tedious and the boosting seem to not have any effect. But it might as well be the more economical one.