yep i think thats right you will only get 5psi. u dont need to worry about the extra 2psi cause that only happens after 4500rpm and only if ur at full throttle. also at that rpm ur by far in motion and the only way its gonna powerslide in the wet is if you want it to. the car doesnt decide out of the blue if it wants to break traction, its ur foot that decides, and thats connected to your brain. as stated before, the car doesnt automatically jump from 0 to 5psi unless you want it to. like i said it all comes down to your foot and accelerator control, heck i could drive my car with minimal to no boost at all and still get around fine thru throttle control.
i personally think this is the wrong way of goin about gettin to know your car and driving experience. what happens when you get used to driving the car with no boost and then one day decide to switch it back on and have it bite you in the ass at the wrong time cause you werent expecting it. just drive it with the stock boost in the dry and cautiously in the wet, practice your throttle control and you will learn thru experiences that the car will and wont spin its wheels at certain rpms/boost, how to get out of it by gently easing off the throttle. u'll also learn not to panic the next time it happens plus the knowledge of why it happened in the first place.
my 2cents