Edit - Remove it from the game entirely
Unless the developers intend to have spell casters spam level 1 spells upon depleting their SP reserves, it would be better for casters if the Echoes of Power scaled with the levels. It doesn't have to be absurdly high to the point that they're practically getting SP potions, just high enough that they can actually be useful.
Items of Power (1 - 10) would not influence how much mana is regenerated as the echoes should ignore boosts given by enchanted items (only the core power of the caster is factored, nothing else).
IE -
Levels 1-5 - What we have currently
Levels 6-8 - regenerate to 10% of their current SP pool
Levels 9-10 - regenerate to 12% of their current SP pool.
Levels 11-14 - regenerate to 14% of their current SP pool.
Levels 15-17 - regenerate to 16% of their current SP pool.
Levels 18-20 - Up to 20% of a caster's current SP pool is regained.
Please note, that this need not be an automatic thing - If balance is a concern, set it so that it requires enhancements/feats in order to increase the amount of mana that returns via Echoes of Power. If that can still be considered unbalanced, then why not simply remove Echoes of Power altogether (seeing as it wasn't well-received in the first place)?
Also, if anyone is totally against Echoes of Power, feel free to use this thread to speak up - the more, the merrier; I got by in the game just fine without Echoes, and so did everyone else.