Mass energize gives Speed, but in G4-5 ( I don't remember for G3) it doesn't increase your max AP anymore.
You are right about experience and items, you lose Spectral boots (5% armor, 5% energy res., 5% acid res., 5% energy res. of creations, 5% acid res. of creations).
But, remember that you can do this as soon as you get to Mera which is right after the Presence events, which is very early game, so for me it's a small trade-off, considering you get one of the most powerful charm : +2 int +2dex (-2 vit...), and a solidified flame that allows you to make +3 mental +3 battle +3 blessing gauntlet almost at the start of the game lol