There is more to getting a harder shift than just changing the line pressure.You can actually get a firmer shift without touching the main pressure spring.This is done by altering the oil flow to the clutches and bands for a quicker apply.Check out the c4 build levels down or on the next page ,lots of good info,or check out the archives.There is an article ther with some valvebody mods to firm the shift for $0.00.
The line pressure is raised mainly for race use .
Increased line pressure will make a tranny shift harder. Automatic transmissions use an orifice usually a hole drilled in the separator plate to restrict flow to clutches and bands to slow down the apply rate. Higher line pressure will force oil thru the holes at a faster rate.
The pressure regulator can be shimmed and line pressure will rise.
OK harder shift is the up side, down side is tranny shifts harder but when put in drive will be in 3rd gear by 10 mph because the increased line pressure increases the govenor pressure and the shift valves move over more earlier. I do some hydraulic design work and after doing some and thinking about auto trannies I am amazed at the amount of thinking and experimentation there is in the valve body of a tranny.
Put a shift kit in if you want harder shifts and when you do all the mods to the tranny make the valve body full manual and crank the line pressure to the sky.
I have been using the B&M Trans Pack which is the 3 stage more involved shift kit compared to their "Shift Improver" cheap kit. Trans Pack comes with stiff inner and outer pressure reagulator springs and in the "strip" stage it uses both stiff inner and outer springs for 200PSI mainline pressure and yet the trans won't shift into 3rd in "automatic" mode until the RPMs are up around 4500-5000 and then it would shift into 3rd. I hooked up the modulator valve which the kit says not to hook vacuum to with that stage but it works but if you come to a stop, it will stay in 3rd if you don't over throttle the gas which made my 4500 stall seen very loose starting out in 3rd but was very easy to drive without continually shifting in city traffic. Shifted very hard, trans shifted instantaniously when shifter shifted into next gear, barked the 28/10.5/15 MT ET Drags shifting into 3rd when running 9s, and remained a foreward shift pattern. I was happy with that kit and I used it in all the C-4s I built for other people and myself too. Just an opinion about the B&B Trans Pack in case you're interrested. I like them.