I would use 3" to/from the spa and 2.5" around the spa after the 3" splits into 2. To keep jets at full strength, you want to reduce head loss in the rest of the plumbing as much as possible. Plus upsizing pipe should not cost you much. For a gunite pool, the wall depth is at least 8" and sometimes up to 12" thick so plenty of room for the pipe.
I put together a head loss model for the two loop system:
ReadMe Version:,v028 Disclaimer,These spreadsheets are provided as is without any guarantees or warranty. Every effort has been made to produce reasonable estimates with these modeling tools. However, unknown measurement errors in the CEC/APSP/EnergyStar data may significantly affect the results...
docs.google.com
There are two cases for each loop. A 3/8" jet and a 7/16" jet. For loop #1, I assumed the bypass is on for a lower head loss configuration.
Loop#1 3/8: 18.3 GPM/Jet, 2.1 lbf
Loop#2 3/8: 18.0 GPM/Jet, 2.0 lbf
Loop#1 7/16: 22.5 GPM/Jet, 2.3 lbf
Loop#2 7/16: 22.0 GPM/Jet, 2.2 lbf
The 7/16" jets would feel a little bit stronger but not by a lot.