Yes a comp. test should show if its really bad. I've had broken lands (couple cracks) and the comp. show the same as the other cylinders but when I did a leak down it showed up. It just depends.
You would think that if when you had the head off and turning it by hand you could see a scrap from the broken land, but I have seen several times where it showed nothing and when I pulled the rod and piston the land would just fall off.
Its crazy stuff. You can make a cheap leak-down tester from harbor frieght (don't laugh) Buy one of there comp. testers $20.00 and cut the hose and hook up a coupler to it and hook it up to a air compressor or (buddy tank when at the track). Put it on TDC and use your ear for leakage. If its a ring-land you will hear it at the crank case ventaltion area or valve cover. You just wont get a % of leakage is all. Well enough said for now.

 

Did you say your ignition timing is set at 20 degrees after TDC?? Be careful of too much ignition retard, it actually looks like your detonation may have coupled with preignition there..not sure how familiar you are with preignition or what causes it, so sorry if I am preaching to the choir..but if not:

Preignition often happens with excessive ignition retard..Basically, since by retarding timing, you are asking the spark to come later in the combustion cycle (starting the fire later), the burn happens closer in time to the next intake valve induction of the cycle..You are pushing the envelope between optimal timing and excessive timing retard at 20 degrees after TDC..

In worst case scenario (which I can see you are aware what that looks like), the ignition being retarded so late in the cycle causes a chain of things to happen:

1. With the burn being so late in the cycle, you have combustion so slow that as the exhaust valve begins opening, the burn is still going, consuming fuel..This increases the temperature of the exhaust valve since the fire is racing out into the exhaust manifold..The exhaust side of the combustion chamber also gets hotter, as does the piston crown..
2. Then as the intake valve opens to let the fresh air/fuel mix in, all these hot parts heat the intake charge, causing the charge density to be lower..At best, this will just lower power or cause detonation..At worst, the combustion is being continued so late on the downstroke that the piston crown, exhaust valve, and combustion chamber haven't had enough time to cool before the fresh air/fuel enters the cylinder..This retained heat is enough to get combustion going long before the spark plug fires again, which causes pre-ignition..Pre-ignition is very much like a blow torch that can melt plugs, piston crown, valves, etc (you get the point)..

If this is the case, you may want to consider retuning your nitrous amount to optimize your ignition timing..

As I'm sure you are aware, more N2O with excessive ign retard can create less power than a smaller shot with optimal ign timing..

In all of my setups, I judge how much boost (or spray for your matter) for a happy medium with ign timing, and I see a lot more power that way, since
with less ignition retard, the piston is still at a point of moving slower towards BDC; the combustion doesn't have to chase the piston down the cylinder as much, so the pressure in the cylinder can stay higher: Higher cylinder pressure = more push on the crankshaft = more hp..

With a setup involving a lot of ignition retard, the piston is already at a point of speedy acceleration down the cylinder wall, so the combustion is chasing the piston down the cylinder wall creating more heat as it goes; lowers cylinder pressure: Lower cylinder pressure = less push on the crankshaft = less hp..

SORRY for the long explanation, or mumbling if there is any
Good luck with your setup ..