I think you can get by on a lot less flow but you need a good and accurate way to measure bare minimum entering water temperature and leaving water temperature. If your entering temperature is 58F then I would should for 1.5 GPM per ton. You should be looking for 5.25 GPM in stage one and 7.5 GPM in stage 2.
Work on getting both stages functioning first as Steve describes. Then you can work on plumbing to optimize the system. On stage 1 you will be shooting for 28k - 32k BTU/h being removed from the water. Absorbed heat = (Entering water temp - Exiting water temp) * 60 * 8.35 * GPM.
I have run as low as 1 GPM but output of the unit dropped down to a little less than 3 ton at that time. With an entering water temp of 56F the exiting temp was around 38 which I wouldn't really go below 40F. On the low end of my pressure tank setting now my flow is around 1.35 GPM and I still have a delta of 11F on the water entering at 56F. Your brother's entering water is a few degrees warmer than me and we live on the same latitude.
Is he enjoying the cold snap? I moved to this house 4 years ago with the Geo being installed in 2008 I think. When I first came here at a setting of 68F on the thermostat it would run 24 hours a day and use some backup heat in weather like this. It was never really comfortable at that temperature. As Steve say most installers just install and go. If it puts some heat out they are done. Yea it worked and on cold days it used backup but I have proved it isn't needed if the whole system is optimized.
Since I have sealed air leaks and optimized the geo and now it hasn't used backup heat in a couple years. Even with the negative temps the night before I still only ran around 16 hours yesterday. The house is 2400 Sq ft and I do just fine with a 3.5 ton.
Can you describe your plumbing more? How many hp is the well pump? Over 3/4hp you need to watch your cycles. 3/4hp and less I wouldn't worry about it unless it is on and off every minute or something stupid like that. Do you know the piping from the pump to the house? The size of piping to the geo unit? You said you have a 30 gallon pressure tank what are the dimensions on it or model number?
I'll gladly give you feedback on what I have done over the years. I need to work on attic insulation next as it can be increased to help some more. Once I get there I will look at getting a 2 stage unit and possibly downsize to a 3 ton as I think I can get there.
You can see my runtime just over this week. A two stage would help me with nice long run times on those small use days.
