In terms of battle power alone, I'd say that Jiren is definitely above Merged Zamas. In the manga, Gokū, when using SSJ Blue's full power, fought on-par with Merged Zamas, immortality and not needing to contain his aura any longer being the advantages that kept Merged Zamas ahead of Gokū. This effectively places Merged Zamas below Beerus, as Vegeta could not defeat Beerus even with SSJ Blue's full power.
Jiren's battle power is described by Top as exceeding even that of Belmod, who, thus far at least, can be assumed to be approximately equal to Beerus, more or less. This does not confirm that Belmod has a higher battle power than Merged Zamas, but the implication is there; at the very least, Beerus' statement that Vegeta could be the Destroyer Deity of another universe places Belmod at least in the same ballpark as Vegeta using SSJ Blue's full power, if not higher. From there, Jiren is above Belmod, so I'd say that, no matter which way it's sliced, in the manga at least, Jiren is above Merged Zamas in terms of battle power.
The issue, of course, is Merged Zamas' immortality and later his splitting into multiple copies of himself. On that topic, I am not informed enough to have an opinion.
"Reign supreme? In your dreams; you'll never make me bow.
Kick my ass? I'm world-class and Super Saiyan now."
I BURN - Jeff Williams feat. Casey Lee Williams, RWBY Volume 1 Soundtrack