Not too long ago, somebody here on the forum wondered about the viability of Sidious with relics. Sidious was on my to-do list by virtue of having the sith tag, and about a week ago I finally got around to upgrade him, and have played around with him a little since then(no not like that). Anyway he's only r5 so far and will stay like that for a while, in addition to still having pedestrian 5E mods, so he's not yet reached his full potential.
That being said, he turned out to be a better investment than expected; he does indeed gain a lot from the relic stat boosts. Damage-wise he went from being the OG wet noodle, to actually landing some decent crits. Around 25k on his basics, which is perfectly decent, at least considering where his damage was pre-relics.
His real strength, comes from the added survivability; with some modded potency, he gains incredible amounts of health because of his unique, especially under Traya and Palpatine. With the added armor, he actually becomes very tanky. In addition to that, there are the debuffs he applies, his healing immunity on basic becomes much more significant with his improved survivability and damage, but his AOE dots might actually be his best quality, allowing for some splendid synergy with a Traya led sith team.
At g13, his aoe will almost always critically hit, applying 2-3 stacks of DOT on each enemy. Now, let's take a look at Traya's lead:
As it turns out, (and this wasn't apparent to me before) every time a DOT pops on an enemy, your sith friends get healed for 10% of their max health; with Sidious applying 3 stacks on each enemy, that's a potential 30% party-wide heal for your sith, each time one of those enemies take a turn, effectively doubling up the heals from Traya's lead, healing your team both on enemy turns, and your own turns when you attack a debuffed enemy. This is actually pretty huge, and yes it does work in practice.
So there you have it, between his massive survivability, debuffs, and high base speed, Sidious goes from practically useless to decent, and even good in the right setups, with relics. By no means should anyone go crazy relicing Sidious, but if you have a decent sith lineup, then taking Sidious to g13 and beyond is actually a good investment.
Here's a rundown of the stats of mine own Sidious. And keep in mind he still has his mediocre pre-g13 mods, there is much room for improvement.