eventually it would peak and not let you go over a certain ppm as long as you have any bit of surface agitation, so I doubt you could get it over 120ppm if you tried.I was wondering if you could have too much CO2 in your tank (aside from creating carbonated water). If you don't have fish you have to worry about would you get faster plant growth at 60ppm then you would at 30ppm assuming you have sufficient lights and ferts. Is there an upper limit where it won't really help anymore? Does it really make much of a difference as long as its 30 or high?
if you have no critters it wouldn't be a problem. what you may want to consider is any wood you have in the tank will start to dissolve (there is a better word to use than that, but drawing a blank right now) quicker with the higher acidic water and will quickly turn your water yellowish with all the tannins released. other's hopefully can chime in on any possible issues with tank silicone degrading from the higher acidity... if even an issue at all.
fyi, I keep my ppm at 60 and my tank is loaded with fish and shrimp. no problems over the past 2 years.