Yeah but they will overpower them by superior stats and systems or class (destroyer\cruiser). The basic problem is that the increased OP is percent based so you get less with levels if you start with lower base OP.
About the OP being percentage based. That's true. But the actual effect of the OP (OP being relatively lower, except in the Foxhound) may be higher on these modded frigates than on vanilla frigates, because it can be spend on more and better weapon systems. While the vanilla frigates may only spend it on extra flux and perhaps a few hull mods.
If I increase the OP further, these modded frigates will be as strong as, or even stronger than vanilla frigates, right off the bat. Which is not what I intended. They are basically "modified" ships, with a lot of potential for the future. They are not necessarily immediate "upgrades" over vanilla ships.
Also look at the hull percentage. When this is increased these modded ships will become a lot more durable than their vanilla counterparts, because of the higher starting numbers. For example, the Sentinel has only 25 less armour, but 500 more hull than the Vigilance. This will increase greatly when a skill with +% to hull is picked.
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Also, the actual effect of percentual increase where the OP is concerned is neglectable, since the base values aren't very high and the differences between them are small.
Sentinel: 25% of 32 OP = 8
Vigilance: 25% of 35 OP = 8.75