It will vary depending upon the mixture. Max allowed is 175 RPM, so 150 is OK and close to normal. Sometimes leaning will lessen the drop. The key is that the engine is still running smooth, which assures you that all of the cylinders are firing.

Vic

Ignition question
 

I've got the same retard mag on the left like you Bob, with Lightspeed right. No Slick Start. I've started the IO540 for the first 12 mths (100 odd hours) with both left and right ignitions on. Have the same individual switchs, and have had absolutely no issues regarding kickbacks. I recently contacted Champion (Slick makers) their advice to me was that there was vertually no chance of the UNBOOSTED retard mag firering until the engine had reached started RPM, and then it would fire at the normal 25*. So because the unboosted mag was not contributing and the electronic was effectively doing all the starting then I should consider starting only with the electronic on, I've now adopted that routine and it's working well. Your RPM drops are consistent with what I'm seeing. Cheers from Western Australia

...and if you can see all EGT's, if they all increase, all plugs are firing.