IMO 10/20 lash is too tight . The engine will lose compression and thus power. I set mine to 13/23 hot. It even idles better, and the power came back.
If power is suspect, the very first go-to should be a compression test, followed almost immediately with a LeakDown test. Of course the lash needs to be adequate.
Also, in my experience those slanty rocker arms are a lil soft and the stems wear a little grove in them over time. You need narrow feelers to get into the grooves,and adjust them properly.
Unless the ignition power-timing is waaay short, you won't find a lot of power there, because......... most slantys run hiway gears like 2.76s, and so your all-in timing may not arrive until well after 30 mph, in first gear. And you likely won't find missing power in the carb either.
The problem is if you only have 105hp, are you gonna notice 5 more?
But the compression test will point you in the right direction, and even more so the LD test.