I'm interested in finding out growth rates of wild Neps as opposed to their greenhouse grown siblings too. One of the problems is that you'd have to compare several plants of the same species to get a benchmark. For example, I have 2 jacquelineaes that I bought at the same time, same size, years ago. One is at least twice the size of the other. If that was my only jac, I'd say that outdoor growing is the way to go. But that wouldn't be exactly true. So I think you need to compare multiple plants and species to get an idea.
I can compare my whole collection together though. Plants like ramispina and sanguinea have gone into the vining stage in a few years, where similar sized plants of other species are still rosetted. Most people would agree that ramispina and sanguinea are easy, vigorous growers compared to lowii and macrophylla. So you can get an overall "picture" of plant growth rates. And yes, lowii is one of those slower guys that takes time to mature. You don't see too many 12" lowiis or large rooted cuttings available for a reason!
Maybe someone like Rob at Borneo or some other growers with access to "wild" plants could shed more light on this.
Nepenthes Around the House