Keeping in the spirit of E6, I would tend to hard-cap metamagic effects (including those from items) at 3rd or maybe possibly 4th level spell slots, which would put the big metamagic effects right out. I don't know what the ideal spot is, but I certainly wouldn't let the metamagic stack to 9th.
Thanks to everyone who has replied to my original question. The truth is that I love E6 and am running an E6 game right now (with some heavy modifications). I find that it opens up a lot of doors for character creation and actually enables certain interesting builds that just aren't possible in standard D&D.
Limiting the effective spell level for metamagicked spells might make sense, but it means banning a lot of feats (some of which were designed for E6 in the first place). For example all of the swift metamagic feats.
As for banning Arcane Thesis, it's mostly used in my example to boost the CL, and there are other ways to do that. Without AT things don't look much less potent -- even if this would prevent CL from going above 9th, you still get:
1. Swift split, sudden maximized, spellshard-empowered scorching ray (3 rays doing a total of 108 damage on average if they all hit)
2. Swift quickened, spellshard-empowered scorching ray (2 rays doing a total of 78 damage on average if they all hit)
3. 3 Conviction Points lets you cast a third spellshard-empowered scorching ray (78 more damage)
That's a total of 264 damage on average in one round. NOVA!
I guess you could rule that since there is only one quickened action allowed in a round, you can't swift twin and swift quicken in the same round, but with some investment of feats you CAN get sudden quicken which will enable all of this.
A few possibilities:
* Ban all of the sudden metamagic feats, considering them rolled into or replaced by the swift metamagic feats (not a bad idea really).
* Limit metamagic's ability to raise effective spell level in some way (double the original spell's level, for example). I'm not so sure about this one.
If you ban the sudden metamagic feats then the best you can do is a swift quickened scorching ray empowered by a spellshard, then followed up with another spellshard-empowered scorching ray and then another one cast via 3 Conviction Point expenditure. If Arcane Thesis is allowed, you can memorize three split scorching rays, then cast all three empowered via spellshard (one swift quickened, one normal, and one more with 3 Conviction Points) -- these will deal 156 damage each on average, for a total of about 468 damage in one round (if they all hit). If AT is banned, you can't split the rays and this drops to a "mere" 78 damage each, or 234 damage total (if each ray hits).
So banning the sudden metamagic feats makes this harder to do and do less damage but the potential for NOVA is still there.
Any comments or suggestions for limiting this further? As a DM I find it hard to just say "no it doesn't work" without having a systemic rule of some sort to base my decision on. I think 234 damage in one round is a bit excessive for an E6 game, although to be fair one would have to compare to what a twinked-out barbarian could do in one round as well.
- Ron ^*^