Some of it has to do with the module stats themselves.

For example, plates cost very little, and provide a lot of HP. The gal scout is able to capitalize on that, but the real problem in this instance is not its ability to take advantage, but the inherent issues in the modules.

They do have significant advantages unfortunately, but I think if we fix everything else first, seeing as how things like plates can be problems elsewhere too, then you can more easily see the problems directly related to the spooks.

I think the assult scout is an entirely viable playstyle. While most of my fits have between 200 and 300 EHP, I also have a number of 'tanked' fits for assault work that range from 400-550 EHP. Because sometimes that's what I need. I'm still a low HP suit, just not 'look in my direction and I die' low.

% increase on select modules while retaining a straight bonus on other modules increases options, while making all modules % based decreases options. The loss of diversity is a bad thing.

To elucidate, have some maths.

Using current armor modules and the GAL scout and sentinel suits to compare.

GAL scout
base armor: 162.5 (130*1.25)
low slots: 4
theoretical max: 756.5 (162.5+4(135*1.1))
increase: 465.5%

GAL sentinel
base armor: 656.25 (525*1.25)
low slots: 4
theoretical max: 1250.25 (656.25+4(135*1.1))
increase: 190.5%

In this situation, the GAL scout goes from having 1/4 the HP of a heavy, to having 6/10 of the HP of a heavy. I think most people could reaonsably agree that 750HP before including shields (232.7 stacking complex extenders) may be excessive on a light suit.

Now, using % increase module only (and modules giving a 20% increase on base total only to keep it roughly in line with the current heavy increase, and no stacking penalty or skill bonus).

GAL scout
base armor: 162.5 (130*1.25)
low slots: 4
theoretical max: 292.5 (162.5+4(162.5*0.2))
increase: 180%

GAL sentinel
base armor: 656.25 (525*1.25)
low slots: 4
theoretical max: 1181.25 (656.25+4(656.25*0.2))
increase: 180%

In this case, the scout remains at 1/4 the health of the heavy.

As you can see, putting armor modules on a scout at all becomes completely useless, regardless of the type of module. Not only does this remove what is a reasonable playstyle through over-nerfing assault scouts, it additional removes the purpose for any other scout to fit armor at all. A 32.5 HP increase is not worth the fitting costs and speed decrease for a complex module, while fitting any lower tier module results in effectively no increase. Thus reducing fitting diversity for the class as a whole in the attempt to try and balance a single fit type.

Now, on the other hand, have a look at a situation where fero and reactive plates retain their straight bonus, while the plate module gains the % bonus. Example just using complex ferro to maximise HP.

GAL scout
base armor: 162.5 (130*1.25)
low slots: 4
theoretical max: 492.5 (162.5+4(75*1.1))
increase: 300%

GAL sentinel
base armor: 656.25 (525*1.25)
low slots: 4
theoretical max: 1181.25 (656.25+4(656.25*0.2))
increase: 180%

While there is still a significant jump in the EHP of the scout, and it's worthwhile to fit armor, the change is no where near as significant. The GAL scout goes form 1/4 the HP of the heavy, to having 4/10. This reduces the ability of the scout to hard tank, while not removing the ability entirely, and additionally makes fitting alternate armor modules (ferro or reactive) worthwhile, which currently are next to useless. Scouts can still play as assault, but they will be properly light assault.

This would seem to me to be a more even handed solution, which does not disadvantage all scouts for the sake of a few. The numbers on the ferro and reactive plates may need to be reconsidered, but leaving them as a straight bonus makes them useful to lower HP suits, while high HP suits can still properly tank by making use of % based armor plate modules.

In effect, create a middle ground instead of pushing suits to extremes.

And just as an addendum, I deliberately didn't compare the Assault/Medium suits in this post. Due to their relatively low starting numbers, they are inherently unbalanced against light suits when it comes to HP. Using the above maths and the current armor plate modules, a GAL scout can reach 9/10 of the HP of the GAL assault. There is limited reason to run the GAL assault as it stands, and a change to % modules will harm them in exactly the same way as the scouts, as their base HP are close enough to the scout to make almost no difference.

Nerf the Dampning a bit. No one can scan them with 2 complex Dampners, OP, it's OP! Make it to they have to fill ALL their low slots, AND have their cloak active to remain off the radar 100%, no chance of being scanned. However, with no cloak a fully Precision equipped Proto Cal Scout or a Gal Logi can scan you. There- balance achieved.

Cloaks are not the problem, again that dampening is!

The smartest players know, that Tacnet 100% stealth is KING. You could give these guys only 1 HP, with that crazy Dampning, and they'd still do extremely well.

The scouts do NOT want this nerfed, so they keep trying to misdirect the attention. But again, the Gallente scouts too low dampning is the problem.