The difference between 15 and 25 ppm CO2 is almost double, and so if every needed nutrient (N,P,K, + traces) is available with abundant light (2-4 wpg), you can double the growth rate of plants (approximately). But at the same time, you can double the growth rate of algae if it has the upper hand, or other nutrients are lacking or in excess.
Until you get a handle on your fertililizer dosing, it's better to stay around 15 for a few weeks. Otherwise you may be juggling too many variables at once.
Extremely fast plant growth can have other issues you may not like that much, like needing to trim overgrowth every week. Or someplants grow so fast they over shadow and inhibit slow growers like crypts.
At levels in excess of 30ppm there is actually some research indicating inhibition of nitrate and ammoinia uptake, so too much of a good thing... would make potent ferts available for algae.
The goal (which I am still working to acheive) is to promote healthy plant grow so they grab the available ammonia, phosphate and nitrates before the algae does.
Good target levels are (according to Tom Barr and others) :
kH 4-5 deg,
CO2 15-30ppm < work on this first
nitrate 5-10 ppm
phosphate 0.4 - 1
Iron 0.5 - 2 (tough to measure accurately)
he recommends people start simpler with lower light say 1-1.5 wpg, and natural CO2 or lower levels to enjoy low maintenance tanks. I of course being a beginner also (to plants), completely ignored this advice and have a grab bag of issues I've just gotten bak on top of but am now finding the chores of pruning and replantng to be taking about 1 hour/week. So I've just started a low maint tank 25gal with some older established plants (java fern, anubias, and valisneria).
CO2 and light are sort of your gas pedals, push them hard and plant growth acelerates, but like a car, if you are missing or have too much other fuels, the mixture gets too lean or rich and the car sputters. At high speeds it gets out of control much quicker. Also if you overdrive CO2 but are missing in needed macro or micronutrients you will get wierd leaf deformities; curling, yellowing, pinholes, long stems but short leaves, etc. Also other plants may not develop their reds and purples.