I think that wpn cooldown is too high, but I seem to be on the opposite side of the proverbial fence from you two because I'm still not convinced GRG needs these extra buffs.. So I don't want to beat a dead horse, but I'd like to try and understand what standard you're comparing GRG to..
In my view, its DPS compares well vs several other abilities even without the debuffs.
E.g., Cleansing Brilliance, an ultimate, is currently set to 3000k dmg every 120 seconds for 150 AM, or 3k dmg over 6 seconds per shot. Lvl3 GRG isn't quite as front-loaded, but it can hit for 3600 dmg in 12 seconds for 160 AM, and it can go on spending that AM to add a lot more dmg..
And what's wrong with it only equalling 18 LRMs? That's a lot. It seems like we have a philosophical difference here because I don't see why we should expect Cap abilities to linearly scale up in power throughout the game. Cap ships have a fixed cost, and it's not like you only get one per game, so they only need to be powerful enough to justify their cost relative to other investments.
18 LRM Javelis cost ~$4,950, 810m, 450c, and 72 supply. As you all surely know, a new Kol only costs $3,000, 400m, 250c, and it uses 50 supply. It's obviously more complicated than that though: GRG can't supply the ~200 DPS indefinitely like LRMs can, cap ship crews require an extra investment, cap ships don't start at lvl 6.. But the Kol also brings a lot of additional advantages with its durability, base DPS, 2 squadrons of SC, and other potent abilities.
And weren't you guys planning on nerfing the LRF dmg to Caps? If you cut anti-medium dmg to caps down to 0.5 (from 0.75), then you'll only be adding to the relative value of caps vs LRF/LRM DPS blobs. (And I think Darvin suggested anti-light dmg vs caps as 0.25, right?)
With the speed debuffs + dmg, GRG does compete with Ion Bolt imo. Ion Bolt has clear advantages (i.e., interrupts, dead-stop/disabled target for 3/5/7 seconds, and longer range). But GRG has a longer duration, better RoF/cooldown, lower AM cost, and I don't think you should discount the value of an extra 3600 dmg in 12 seconds (i.e., 3x GRG shots for only 120 AM) when you're trying to take down a fleeing cap ship. With 65% mitigation and 10 armor on a target, 3600 dmg amounts to an extra 840 HP off the hull of a (presumably) wounded ship. That could easily be the deciding factor when an enemy cap is trying to escape.
Also, compare that to lvl 3 Nano Disassemblers. The armor penalty helps other ships (like LRFs) add more DPS, and ND does add a lot of dmg for its AM cost (625/875/1125 pure dmg), but it's spread over 45s, which gives a lot more chance for a target to escape and/or get repairs from an ally.
Finally, I want to point out that the 20/40/60% wpn debuff is 2x what's provided by Vertigo (10/20/30%). Vertigo also only lasts for 25 seconds, and it costs a bit more AM. It does have an accuracy debuff as well, and it does target an infinite number of units in a small(ish) area, but it doesn't have the anti-cap assassin power of GRG's high DPS and speed debuff, and it has a longer cooldown.
Having said all that, I'm not completely opposed to your last suggestion.. A scaling duration of 10/15/20s seems ok.. (I was considering 8/12/16 duration at one point, along with a cooldown of 12/9/6).
I do think the 60% wpn cooldown debuff is a bit on the extreme side, and I'd rather see it scaled to 10/20/30% at most.. I also think you should still consider a range buff (maybe up to 6500 or 7000), since that would help it reach fleeing ships and help it snipe targets (e.g., to apply its debuff to a starbase) from a safer distance.
But I'm not convinced that this is necessary.. 