That 1000 watt hortilux has at least double the PAR watts of that MH light, i.e. alot more useable light...if you have enough space & plants and want them to grow BIG & FAST..go with the 1k dude..but you are going to have to prune & train those plants for certain as sativa based strains are going to stretch like crazy during flowering so you need to be bending, pruning, & breaking (they love when you just snap the tops over, they grow back really strong from the lower and side branches due to hormonal changes as a result of the breaking of the tops..just let them hang over, they will fix themselves in a few days)...Temps of 88F are fine if you use CO2 enrichment, these would actually be near ideal conditions for a sativa..otherwise it will be a bit warm as plant growth starts to slow and you start having heat stress problems around that range...Maybe look into an exhaust system or A/C lowering, etc and try to get those temps down..either way more light = more growth = more yield. But as you know = more heat, more electricity. You will likely yield twice as much as the 400 MH with that hortilux bulb.

Look at it like this:

400 watts of light : 4 x 4 ft space (16 sq. ft) = 25 watts per sq. ft.

1000 watts of light : 4 x 4 space (16 sq. ft) = 62.5 watts per sq. ft.

For ideal growth rates and flowering results with rock hard buds and heavy trichome production you want to be in that 40-60+ watts per ft2 light range with HID lights...for vegging 20-30 per ft2 would work just fine though if you decided based on temp problems that's what you needed to run.

How big are the plants now? How many? Any pics?