I use the rule what ever is cheapest per stat. It's better to buy something that's twice as much if you get 3x the stats
The addition of land has always been the single most largest increase in plunder. You can't build anything but T6 on ICE lands. The Colony L1 is very similar to a COE at L3 but all those answers and more are on Wulfs build thread. So I think your answer is yes they T5 L3 vs a T6 L1 would be similar in plunder. The exception would be your high def stat attack building and their 5% additional plunder at L3. The Torture Garden has massive plunder @ L3 but it's marginally better than the Titan. It's more the damage or def power were the next tier level come to play.
I bought all land up to 45b on HF. Now I place the attack building one by one. When I'm done ill go back to T5. The golden rule of builds do what's cheapest first with most benefit. But to answer ur question I get an extra 1.5m to 2m a hit with every T6. But it's double pay eb so half that. I've no T5 to compare with.
My next Highland is 90 billion gold. My next Hoarfrost land is 15 billion. I have 5 Hoarfrost lands and 15 Highlands. A T6 level 1 is weaker then a T5 level 2, but not by enough that you should debate overly about it. If I had ignored the Hoarfrost entirely and kept on working at upgrading my 14 T5 left to go, I'd have better stats, yes. But I'd rather get some T6 down, mainly because I'm bored of the Highlands and these Hoarfrost are new.
So, I agree with the idea of building a few T6 then return ro highlands. This was WAY WAY WAY more beneficial for me. Because, I had all HLs and I have I think 15 volarys left to get lv 3. that's about 1.5T and the stats increase would only be like 200k or some crap. but instead ive spent I think more than 1.5T but im not exactly sure. but its probably really close if not more and I have gained like..2-3m cs. as well as like 6m extra money per attack. but soon im going to just pour money into allies. Then HLBC at like Christmas~ish I think.