Alky/water injection can be a big help at reducing intake air/combustion temps in a boosted applications, especially in non-intercooled applications. And I've seen some applications in VERY high compression naturally aspirated applications. But for most of the milder street stuff (10.5:1 or less) it probably won't make much difference. You gain about 1/2%-1% HP for each 10F reduction in intake air temp. But unless you can control mixture, you may lose with a rich mixture what you gain in temp. reduction.