let me know what you think.

They do look extremely powerful.

I'd tend to suggest applying unique special rules to just the Phoenix Lord's Unit, rather than the whole formation.

Do they have a rule in 40k that boosts the power of every one of their aspects on the table?
I'm not overly familiar with the Phoenix Lords.

I also need to point out that the closest Epic equivalent to an entire 40k game is a single Engage action by a formation. 40k rules that effect the whole type of unit in a game have a dubious equivalence at Epic scales. Leader special ability and that have some pretty fearsome FF/CC values, as appropriate, would probably be better IMO. I have NO 40k knowledge of the PL and their abilities mind you, just pointing out the closest "like" thing between the scales as a useful mental model (as described by Lord Jervis in the original designer notes in the EA book)

But doesn't this just reduce them to the level of an exarch? How fearsome can close combat stats really be before they become like chaos beserkers? At the moment most of the buffs that the phoenix lords give are pretty engagement focused so would match up well with 40k = an engagement.

Hmm, this provides an intruiging set of ideas - obviously onyy for 'fan' or fun games rather than tournaments.

I think it might work as a concept because I think the Eldar lists will tend to be self-limiting for the following reasons:-

1. Eldar generally work best when they have an activation advantage, and the use of aspect formations tends to work against this. At 300+ per formation the actual number of character led aspect formations will be limited.
2. Monotone aspects are generally less usefull, and so the impact of character led formations will be reduced
3. I would suggest that the cost of the character is 50 points and must replace an Exarch (so +75 points per character). This will further reduce the number of such characters.
4. I note that you have wisely refrained from providing a Warp Spider character - these are already the best aspects and have no need of an additional upgrade.

Have you actually tried using them in an Eldar list?

Oh sorry I wan't really making myself clear there. My intention was that the benefits should be less passively for the entire formation always and more combinations of special abilities (preferentially to the the PL stand itself) and more targeted to applicability to being used in Engagements as opposed to just always ON. Judicious use of appropriate FF/CC stats should be used to balance these abilities and represent them as a powerful stand so you don't go overboard, that's all .

Good idea that I'd like to see some more thoughts on to see about getting it fully baked.