Thank you lloydo, peekay123, bko, and lucb. In answer:
User Code: same for Photon and P1 (with exception of P1 spare pins used on P1)
Question: if in SYSTEM_MODE(AUTOMATIC), does user code run prior to WiFi acquisition?
PCB Design: no copper under P1 recommended keep out zone:
Antenna: have not tried known-good antenna with u.fl connector
WiFi Received Signal Strength:
Samples collected from Photon on breadboard and P1 on custom PCB with WiFi.RSSI() funciton, both connected to same WiFi network, 10 samples in a row taken from Photon, then 10 samples in a row taken from P1, samples taken roughly two seconds apart
Photon on breadboard (dB): -66, -66, -67, -66, -67, -67, -67, -67, -66, -67
P1 on custom PCB (dB): -75, -77, -77, -75, -76, -76, -75, -75, -76, -76
Question: if multiple WiFi credentials are stored in the P1, how does the P1 go about acquiring a specific SSID? Does the P1 assign any certain priority to each SSID based on when the SSID was stored in memory?