I lost 33% on 3 or 4 and the other was not that great either. At first thought it was a HG issue since two cyl. close to one another were low. NOPE...it all was the result of a slight n2o backfire. I thought I busted the ringland.
mine was more of a fuel not getting burned from the loss of comp. I had the heads gone over (this was before they were ported) when I replaced the HG and still had the issue. I wanted to rule out anything except the shortblock. At anything more than 3/4 throt it just dumped black exh. the bumper got a nasty $h!t stain looking buildup on the bumper by that sides tailpipe.
I took apart the shortblock after I pulled it. You know how you kind have to tap out the pistons once undone from the crank? Well the cyl. that was 33% down litterally just fell out of the block after I took out the last rod bolt...fell right out and hit my foot...

was kinda stained from blowby but otherwise not a scratch...I guess the rings just got too wore down.
I replaced the shortblock with a DSS pro bullit 306 after going over machine work, parts, time, assem. hardware, ect. The DSS was just a good deal. I am VERY pleased with the quality and work of the shortblock.
I would check the heads out either way just to be sure...you have to pull them no matter what so maybe pull them and have them looked at before pulling the trigger on a new shortblock...unless your looking for an excuse for a new shortblock then I say be 100% sure its not the shortblock and start off fresh with that as well
Good luck either way.