You're fooling yourself if you think an air/water isn't going to have the same difference in performance between cool nights and hot days. The bigger the difference between charge air temp and ambient temp, the better the intercooler will work, it doesn't matter what type you have. If you don't notice a difference between cold outside and hot outside, something is wrong with your setup.
You're not going to go below ambient air temp with either one, so they'll perform the same if they are both adequately sized. Basically, if they're both big enough, one won't really perform any better than the other. It's just physics.
Don't fall into that trap, air/water isn't more efficient no matter what, that's BS. You can still end up with a core that's too small or a radiator that's too small, or a pump that doesn't flow enough, or an inefficient core, etc., if you don't pay attention to detail. I'm not saying that theres anything wrong with air to water setups, just that making sweeping generalizations about either is dangerous. It's actually difficult to make a air/water match the efficiency of a well designed air/air.
So don't just slap an air/water setup on there and think it will outperform what you have now, because it may not, make sure you design your setup carefully.

I don't blame you for wanting to get rid of that 19x6" core you have now, I'm not a big fan of cores that are that long with that little cross sectional area. I always wince when I see those 30x6" cores with square end tanks on Hondas, they're like a big air restrictor..