kind of depends on the fish. sensitive fish i wouldent do more than 20% every few days at most. more hardy fish you could do a 50% every few days if you really wanted, as long as their isnt a major diference between PH in the tank and the water your adding, and you use a good water conditioner like PRIME
for example: i do a 50% sometimes 75% WC on my 250g tank every week, with all hardy fish. yet when i had rays and a black ghost knife together, i only did around 20-25%.
as for growth, keep up on the water changes, and feed a good variety of foods. staple pellets, then any freeze dried and/or frozen foods.
With my slow growing plecos I change the water 4-5 times a week 20-30% each time. The water is all been aged so there is no cholrine in it. It seems like they grow almost 2X faster. O yea make sure they are always full
At a fish club meeting a few months back, the speaker was an editor from one of the fish mags. He advised, (after a very technical lecture that was amazing), doing LARGE WC frequently.
I am careful with temp and Ph being close with the new water, but I now do 75% WC, and my water quality and fish / plant growth has never been better.