i just went to my lfs looking to fix this very problem for the tank im in the process of building. imo dont use the top because there are water tite caps you can buy for 2 or3 dollars that guard the balast
the acrlic top will not reduce the intesenty but why do you want to use a top it will only keep the heat from you vho lights inside and your tank water will reach very dangrous levels for your corals vho will keep everything alive the acrlic will not efect the intsety at all
It will lose some intensity IMO. Acrylic is very clear but unless you wipe it off constantly there will be water droplets and salt creep to scatter the light.