[...]I was thinking more in terms of getting brewing water quickly when I need it. I don't need 150 gallons a day, but I'd like to have 8 gallons in less than an hour when I need it.[...] GPH is more important to me since I would not likely use a storage tank and just generate it on demand.
I'm in the same camp. I built my RO system for my brewery needs and that's pretty much the beginning and end to it. So brew day requirements are paramount, though I do have the standard 4 gallon pressure tank servicing the RO faucet added to my brewery sink as it's handy for rinsing measurement instruments.
I have a 100 gpd system tuned by Russ @Buckeye_Hydro and brew ten gallon batches. I typically need 18 gallons for a batch (10 of which needed in the hlt to cover my hex, the rest in my bk for strike volume) and run the system straight into my brew rig the afternoon before a brew day. It takes under five hours so it doesn't require much planning.
One key for any RO system is maintaining its health. They hate sitting idle. Running the system daily or at least weekly even for a small amount of product is way better in the long run than only running it for brew day requirements...
Cheers!