Craig,
with such low tds reading, you probably would not need any bigger RO than you have now. Also, it might be worth checking the tds reading as it comes out the RO and before it goes through the resin. It will be so low, maybe 000, that your resin is hardly, if at all, actually being used.
As you already have system in place, I would stick with your present system and it will be much more cost effective in the long term.