How much light is too much?

I have a 60 gallon FW tank which currently has 4-48" standard
fluorescent bulbs over it. As a general rule, that doesn't seem to be
enough for some of the more light-demanding plants, such as grasses
and red-leaved plants. (Though they may also be getting done in by my
clown loaches....)

So I'm thinking about retrofitting a couple of MH bulbs into the tank.
Would a pair of 150-watt MH bulbs at either end of the tank plus a CF
or other fluorescent be far too much light? That works out to over 5
watts per gallon. FWIW, I am going to have a CO2 tank up and running
before I put the MH in if I do it at all.

Thanks in advance,

Bob
Ontario, California