I think I've made an error for over 4 years into adsense. I have my sites setup for custom channels but I've never made them "TARGETABLE" before today. Will this make my income a lot higher?
Logically, yes. Since advertisers can target their ads to specific channels and pay higher $$ per click. A lot higher? Well if you can manage to get 3-4 advertisers bid on the same channel then yeah
Not a lot higher. After it depends on your sites traffic and value. If they're good, then advertizers will want to show on specific blocks. But in the past advertizers could target any site anyway, it's only after adsense introduced targetable channels for specific blocks. But advertizers can already target your site and pay by CPM.