Bigger sumps means more water which means better coral growth. Can only be good.
The lighting will depend all on what corals you keep.
The lighting power comes down to which bulbs you use and ballast.
T5s give you a good spread of light and IMO more tubes the better.
Most t5s will lose the output within 6 months , if you get nore tubes then you will get a better spread of light on the sides, so it depends how wide you tank is vs the amount of tubes.
In terms of metal halides I think you can put to much light on tanks as discussed here about a year ago. cant find the link, it was the same time everyone ran out and changed to 400w which i said was a waste of time. But saying that 400w would be ok on bigger tanks as they put out better spread,
My tank runs 250w and 400w and i have found no differance in corals other than the 400w look a little brighter. the 400w is a big bulbs so you get far better spread of light. Also things to think about is not all bulbs are the same. some 250w bulbs are brighter than 400w bulbs, especially the bluer bulbs tend to put out less light. so a 250w 10k would put out more light than a 400w 20k.
If you run a tank that has low nutrients then light is not that important as you will starve your corals if you have to much light. and that is how you get rtn.
Corals cant use the extra light so in a low nutrient tank they basically eat themselves to get food as they need the energy to use the extra light.