As someone who has tried early game Mercury spam extensively, it's not really worth it. The civilian tag affects ship AI, forcing it to act timid regardless of orders/doctrine, and it seems to be more conservative with missiles than usual, too (perhaps simply because it's rarely in range to use them). Your own player levels contribute so much to the XP formula, saving 1.5 DP ( Mercury over kite(A) ) isn't really relevant.
If ship limit wasn't a thing, maybe spamming Mercuries would be better. With 30 slots, there are better support missile boats.
Hmm in my experimentation, the (unofficered) Mercury ship AI does depend on your fleet doctrine; if I set them to cautious then they'll stay back a lot, but if I set them to aggressive they'll be much more willing to get to the front lines and fire their stuff. I used 2 Salamanders and 1 Antimatter SRM for each Mercury, and it seems to be fine.
The idea isn't to just use one of them, but to use a bunch of them, i.e. for 20 DP you can have 1 Gryphon or you can have 10 Mercuries and thus 30 small universal slots to play with. So you could have your usual fleet of ~220 DP but then supplement with 10 Mercuries for example. Granted in this case for XP purposes it'd only shave off 15 DP from your overall fleet DP (which is probably around 500 or so let's say) but at least it's still a bit, along with all those extra weapon slots.
I think in testing, the main problem I found is that each Mercury (naturally) isn't able to take out a [REDACTED] frigate on its own. So in pure AI fights (i.e. without the player controlling a ship nor actively giving commands to ships), without them, my fleet naturally starts moving toward the enemy spawn location as the fight progresses, but with them, my fleet and the enemy fleet naturally scatters out as the Mercuries lure [REDACTED] frigates away and they start backing off since they can't kill them on their own (unless they mass up). So the battlefield becomes more chaotic with frigates (both mine and theirs) all over the place, and cohesive battle lines never form, which makes the battle hard to conduct. Not sure if it means I just need to experiment with them more or something.
I just tried unarmed Afflictor-P for the system, but it would not Amp enemies unless I ordered it to escort an allied ship. So, I put a tactical laser on Afflictor-P just so it can Amp enemies on its own. So much for that idea to cheese civilian status for gain.
I tried out Afflictor-P and it does fine with going forward and Amping enemies, at least under full assault with the fleet doctrine set to reckless. I think the issue though is that it does back off often; it seems to need another ship nearby that's capable of actually attacking, for it to be willing to go in and Amp. Otherwise it just backs off a lot and (like with Mercuries) just leads to a more chaotic battlefield as the ships scatter. Oh well.
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So, I took a look in the actual code for the XP bonus (it can be found in \com\fs\starfarer\api\impl\campaign\FleetEncounterContext.java as computeBattleDifficulty()), and there are a couple more things to add to what I posted above.
The first is that d-mods add a 90% multiplier per d-mod to that ship's DP count for the purposes of the XP bonus, and this is multiplicative for each d-mod. (On the other hand, s-mods have no effect on the XP bonus, so go ahead and max them out.) This means that a ship with 5 d-mods only count for 0.9^5 ~ 59% of its DP. This is without limit (a ship with 8 d-mods would count for 0.9^8 ~ 43% of its DP for example) and seems to apply to civilian ships as well. Rejoice all ye Derelict Operations users, for those people who use both Support Doctrine and Derelict Operations.
The second is that for enemy fleets, the XP bonus calculation is a bit different. Officers are worth twice as much compared with for the player. In other words, for the player, a level 4 officer (gamma) was worth 22.5 DP, a level 6 officer (beta) was worth 30 DP, and a level 8 officer (alpha) was worth 37.5 DP, but for the enemy fleet, those officers are worth 45 DP, 60 DP, and 75 DP, respectively. So for [REDACTED] fleets, close to 75% of the XP bonus will actually be coming from all the cores that they have, even if those cores are on frigates or whatever; a Lumen with a beta core on it would actually be worth more, in terms of the XP bonus, than an unofficered Radiant. So in terms of XP gain, it's better to look for fleets with lots of officers, instead of large (in terms of ship counts or sizes) fleets. The actual ships in the enemy fleet only end up being a small fraction of the XP bonus.
(As a side note, if you try to estimate an enemy fleet's DP count for the XP bonus, note that the formula multiplies the enemy fleet's total DP for the XP bonus by 0.67.)