it absolutely does NOT sputter or flat spot or anything. Its just seemless really.
Maybe with straight water or 75% water/25% methanol it might because if you add too much water it won't burn, but with at least 50/50 I have never seen an issue with that at all.
I have had one small nozzle, 2 small nozzles, one large nozzle, 2 large nozzles, and 2 large and one medium nozzle(current). I have had one and two post turbo, but after the bov and intercooler, and now I have that plus a pre turbo one.
Keep the pre turbo nozzle small and have good pressure so that the atomization is very good. This keeps large drops from hitting your compressor blades as they spin upwards of 120,000 rpm. Remember, liquid does not compress
Also a higher pressure pump is a good idea for better atomization and flow. Plus the larger more modern pumps can handle more flow too, for you who run lots of large nozzles.
My results have been VERY good with this. I can't give you a figure, but I got nearly a full second improvement on the 1/4 mile track when still on the 14b from adding a single medium nozzle(0.7mm Aquamist) post turbo.
Also, with my current setup, I can't run over about 10-12 psi and about 60% throttle or I get knock. I can floor it at 26psi with the methanol.
Also I am running blue bug juice currently, with good results. With no pre turbo nozzle, I had to run pure methanol and only managed max of 22 psi.
I have also considerably leaned out the fuel map since the pre turbo setup as well, and especially the transition area(boost coming on and no methanol/water injection happening yet) It was VERY rich.
Another advantage to running water/methanol is that your intake manifold, intake valves and piston tops will be steam cleaned. the exhaust valves get some help too and so does the chamber part of the head.
With just methanol and richer mixtures, my freshly rebuilt engine, that I painstakingly cleaned the head and pistons, already had carbon flakes/buildup on.
As for on/off, or progressive, I find the on/off is just fine.
Set it to come on when you need it. If you start to get knock at 3800 and 15psi, then have it set to spray at 3700 and 13 psi. For example. It works very well.
Progressive would work fine too I am sure and would give you more options and more to play around with, but if you don't have the money, or just don't need the extra complexity, a good adjustable boost switch works just fine.