Peter, my understanding is that the L17A1 (now A2) was the British manufactured replacement for the L2A2 ball cartridge around 2006 as the general service cartridge in the L85 / L86 SA80 family of weapons. It was loaded with chopped tubular rod propellant like its predecessor, the L2.
The L15A1 (now A2) was introduced around 1999 after evaluation of the Diameco C8 (AR platform) short barrelled rifle for British special forces (which was adopted as the L119A1). Both L15 and L17 use the same SS-109 style bullet. The L15 has ball propellant to optimise it’s performance in the shorter barrel and direct gas impingement systems of the AR platform, with the L17 being better suited for the longer barrel and short stroke gas piston operation of the SA80 system.
It was reported that during a critical shortage in the early 2000’s the US acquired RG 5.56mm ammunition as an alternative to their own. A memorandum was issued by the USMC / Navy Department banning the L2A2 (in which the L17 was based) in US M4 carbines, quoting functionality issues in U.K. MoD trials of L2A2 in M4 style carbines. It went on to say there was no issue using L2A2 in US M16A2 rifles or M249 SAW machine guns (which suggests the main issue was more to do with barrel length than gas system.)
Although L15A2 and L17A2 are both still manufactured, there is probably not a great difference in performance in practice, particularly for the L15 in both short and long barrels. Hope this helps, Pete.