Warframe

• Haste mote is multiplicative with attack speed modding on the weapon while arcanes and most other ability buffs are addative.
• True.
• Also true, though it does have certain synergies that can buff it into the stratosphere.

Worth noting that glaive throws and heavy detonations do not break her passive so you can just aimglide around and nuke with Xoris or Glaive Prime with impunity.

The main thing that makes Wisp such a good support frame is her sheer convenience and low maintenance. Support is a very unpopular and underplayed role in Warframe and for a good reason. All the good top end frames are entirely self sufficient, requiring no external aid to easily dominate. Wisp breaks the mold by providing generally useful buffs that apply to you AND your teammates while requiring no work from the player on top of having a solid CC/damage ability and a free subsume slot. A 400% power strength Wisp with Pillage makes for an incredibly nasty weapons platform frame while also helping out teammates with her Motes. She can support her teammates without needing to work for it all while being capable of doing the mission by herself if need be.

A few things:

• What wisp does for herself she also does for the group. Your teammate can afford to bring a frame that's not survivable enough normally because of vitality mote, and with very little coordination from the group.
• Because of the way reservoirs work, that strength requirement isn't as much of a tradeoff as it seems. Wisp can afford to tank her efficiency into the ground and still function very very well in any mission where you can stay near your reservoirs.
• You compare her to arcanes, but those are a very large farming investment to get to high levels, whereas she drops from a boss fight / relics. If you're one of the players that already has everything the comparison is fair, but if you're looking at a new thing to acquire the two are very different hurdles.

Ultimately she's probably rarely the best possible choice, but part of the reason that she's considered "the best support in the game" is that she does so much with so little investment or effort (and that she was given that title before a bunch of older support frames got buffed to stand on par with her)

Haste mote does wonders for melee. Vitality mote is just convenient. In super endgame stuff you can't rely on it alone for survivability but it is a very serious ehp boost, and life recovery can be pretty hard to come by. She DOES fall off towards the late game, but she's still damn useful if only as a big quality of life boost.

The problem with support Wisp/support frames in general is that if you're already going into endgame content, you can usually expect your team's kits to already be sufficient for that content. You can increase their survivability further by making use of shock motes to stun enemies and rad procs but this typically just slows down missions (especially the shock motes in defense missions) if they're used in situations where they aren't really needed, and can cause issues with builds that make use of enemies clumping together like if you're trying to make use of Gas/Blast or have a Harrow in the squad trying to buff with Covenant. Rad procs from breach surge usually aren't as much of an issue though.

If you are willing to give up fashion, you can subsume Elemental Ward and use a green energy colour for a reload buff to complement Wisp's Haste mote, along with the Everlasting Ward augment mod so the buff is actually useful to your team.

Breach Surge is a lot less useful now that enemies are often protected by overguard, ignoring the blind effect and being immune to the CC effects of Radiation procs, I think they might even be immune to friendly fire too (they aren't targetted, and I think they're immune enemy AOE attacks like a Bombard's rockets). You can still try to make use of it though, there's only so many enemies that can attack at the same time so you can still reduce the number of enemies attacking your team/the objective at the same time.

Pillage/Fire Walker for status cleanse; Pillage gives you Armor Strip at the same time while Fire Walker gives to a movement speed boost for memes. Your team still needs to walk through the flames though but it's a more passive in its use versus Pillage which requires you to cast every time you want to use it. With Overguard being on everything now, status cleansing is a lot less useful, but it was still nice to have for Sorties.

I think you're much better off modding to specialize for your loadout and letting your team pick up the buffs passively instead of trying to directly buff your team.