I was tossing this around in my head and speculated: Would it be too crazy to allow someone to use their lore skill as a free action during a combat turn, granting a +10 on a successful roll against targets related to the knowledge roll? I.E. Forbidden Lore: Xenos, a successful roll lets you know that Eldar have weaker armor at the neck, or something like that?

Something similar is done with the Evaluate skill in Deathwatch, so it's possible, but I wouldn't suggest making it a standard option. Perhaps if it involved use of a Fate Point to take advantage of the knowledge (or if the knowledge increased the effect of spending the fate point to assist with the task) it might work out well.

The concept seems reasonable, but the +10 is too much.

I would lean toward:

A Challenging +0 Forbidden Lore test granting a base +3, +1 per DoS, to either BS or WS (choose one), to you, for the duration of the encounter.

A Difficult (-30) Test would grant the same bonus to you plus one additional member of your team per DoS up to your Int bonus.

A Hellish (-60) test would grant this bonus to anyone under your command. i.e. The army you have tucked away in your holds.

Once you have chosen which attribute to modify this cannot be changed for the duration of the encounter. This test may be used only once per encounter, and only against an enemy for which you have the appropriate, trained forbidden lore skill. Multiple uses of this test by different players will not stack, and any DoF by one player will reduce the DoS of another.

Maybe not as a bonus to hit, but certainly as a bonus to actions like feint where a knowledge of how the dirty xeno thinks would be fine in my book.

I do something similar.

If the players have time to make battle plans (they know the fight is coming) they can make a Imperialis tactica roll to gain bonuses (minor ones based off of degrees of success) in battle. If they know who they are going to be fighting they can make an appropriate lore role to add bonuses to their tactica roll.

I also allow for bonuses from lore/tactica rolls in boarding / hit and run raids etc, again given they have time to make plans and know who/what they will be fighting.

All in all the bonuses are minor and everything the players do the enemy can do as well so it is not a game changer, but it does allow for all the players to feel as though they are a contributing part of the team in a fight.

Knowledge is power. I agree with the Degrees of success idea, that is normaly how I handle the lores. The higher the success the more you know on 10% or lower I let them read a portion of the discription of the mob(not hard stats, but powers ect.)and allow them to discribe it in their own words to the rest of the party.