++[++[+[]][+[]]][+[]] outputs 2 as a Number (rather than as string). How high could we go with those 3 characters ([, ] and +, no spaces) with less-than-expone

++[++[+[]][+[]]][+[]] outputs 2 as a Number (rather than as string). How high could we go with those 3 characters ([, ] and +, no spaces) with less-than-exponential scaling compared to amount of digits of the result, as Number (as string it's easy by concatenating digits of course)? I'm thinking it's not possible without being able to use '*'