I wouldn't go with an XP thing.  But there are other ways to give a bit more added power to frigates and cruisers outside simple tech tree upgrades.

I think since the game engine got the ability to make upgrades to models, adding us an upgrade option to existing frigates would be a good idea.  These upgrades for a small fee would change the frigate into a "variant" of it's model.  So far I came up with 2 variants, but obviously more can be made.

Variant #1) Reinforced: +2 armor(TEC)/ + 5% HP, regenerates 1 hp every 5 seconds(Vasari)/ +3% shield mitigation(Advent)

#2) Fire support: +10% Damage(TEC)/ +10% shield penetration(Vasari)/ -10% weapon cooldown(Advent)

Here is how it would play out in my mind.  Say I'm facign a fleet of HC and Carriers vs my fleet of LRMS and FLAK.  I may want to beef up my flak into armored so the HCs dont remove them as fast and they can provide cover against the fighters while I buff my LRMs with extra damage so they could take down the HCs a bit.  Even though the matchup would not be ideal.

Obviously my opponent would probably counter by getting Fire support variant on his HCs and switching targeting priority to LRMs, possibly docking his strikecraft or retreating them all together. 

I really don't want to do variants on ships.  That and do you realize how much more than +10% damage +10% shield bypass is?  It's incredible.  By the end of the tree, you end up with ships that bypass 30% of the time and the missiles themselves are also significantly more powerful.  Adding another 10% on top of that to assailants is going to make them absolutely pound the Advent.  I'm serious.  There would be absolutely no stopping a late-game Vasari with the PM variants on their Assailants.  None.

Oh, and there is that memory limit too...

I've never played supreme commander, much less know the game mechanics.  So thanks for calling me on lying when I wasn't 

Sorry..  I've been a little touchy lately..

But in all honesty, I really have always wondered why only capital ships got better.  I've never wanted frigates to near capitals in power, but I still want that extra touch of power that an experienced fleet is going to bring you.  It also means that it would reward good players that pull out of a battle they'd ultimately lose before they lose everything.  It gives them an extra touch of power in battle the next time.  That's how things work in real life.  A regiment goes off to combat and is an extremely lossy battle.  The next time they get sent out, they already know what to expect from battle and fair much better.  That's just how combat works.  Experience>Training.

I dont want to see it as just getting more life. In fact you could make the benifit of xp give different benifits to each class of ship, or at least each faction

These are just rough numbers, not final. Just like 10% weapon cooldown could prove even more powerful when stacked with cooldown aura and cooldown reduction in the tech tree. I repeat these numbers are not set in stone, they are there for illustration pourposes.

Yes 30% shield penetration is a lot. But if you are concerned with Vasari getting 40% shield penetration you should know that  Vasari can have 50% shield penetration with phase missiles right now. Subverters increase missile bypass by 20% on targets of their debuff. 

But TEC can reduce shield mitigation by 40% also with their support cruiser so their LRMs can be just as deadly given that their tech tree allows for 30% damage increase vs 20% of Assailants. TEC actually deal more raw damage than Vasari. Because 40% mitigation reduction (assuming 67% shield cap) is equivalent to 121% damage increase (You deal 73% of base damage vs 33% without mitigation debuff). Where as a 50% chance to deal 300% damage (Dealing 100% of damage instead of 33% if missile bypasses shields) is a mere 100% damage increase. But Vasari can destroy  targets with ratio of hull to shield closer to 1 more efficiently because they do not have to reduce the targets shields to 0. If such cases Vasari have a effective HP debuff on their targets, reducing the total off effective hp by certain amount... which is rather hard to calculate.

So to answer your question: Yes I know the difference between 10% damage increase and 10% shield penetration... do you?

Actual reason behind the additional shield bypass buff is so that Vasari ships without phase missiles can get a chance to bypass shields.  And this would not be a possible buff to strikecraft, though the variants for carriers could be more interesting, like a squadron specific buff to make specialized carriers more desirable.

Now Ryat, what is this about memory limit?

Ahhh silly.  Can we get less detailed models? Would that free up some memory?  My idea was not to have 3 meshes but rather a model made up of more than one object.  The upgrade would be a smaller model attached to the basic model.  The attachments could be less detailed, and take up much less memory.  Or if anything simply a different texture for the same mesh :/