If you are referring to the Recording Rate, the RCX3 has a fixed Recording Rate of 5 Seconds. It is not likely this will be changed via a firmware update.
The RC3 GPS has a 1 second Recording Rate.
The RS800CX and RCX5 offer a selectable Recording Rate from 1, 2, 5, 15, or 60 seconds. Their default setting is 5 seconds.
The RS800CX also offers R-R Data Recording for recording Heart Rate Variability at a resolution of one millisecond!
POLAR® USA AMBASSADOR --- "Commit to Transform!"©
• RS800CX PTE, CS600, Spd/Cad/G5 on MARIN® Alpine29er
• V800, M400, FT80/G1, H2 SoftStrap, Loop H7/H6
• GALAXY® S4 Active, QUAD LOCK®, Sports Tracker
• SKIER'S EDGE® QS5 BigMtn RPM
• P90X®, GoFit® PowerTube, Weights
• PERFECT® FITNESS AbCarverPro, PushUpV2 & MultiGym
• CAMELBAK®; ERGON® GP5; LUMINA 650; MARS 3
• K2® BOA MACH100; HARBINGER® 1260
• RCX5, RC3 GPS, RCX3, G3, FT60/40/7, F6, FA20, AW200, M71ti