They claim 30hp. There'* some conjecture on that.
1. They dyno'd a car. Dunno.
2. They made an assumption based on the S2 results from the same ratio change.
3. They got the numbers from the manufacturer, Yella Terra. Not likely. The Aussies never saw the S1 L67.
They are advertised to work with stock springs and pushrods. I dunno. I have valve float now with stock springs (raised shift points). I'd personally suggest stronger springs (stock is 80#) and stronger pushrods. I have a source for the pushrods, and have them sitting right here (came today) but they won't be tested until around February.
In the long run, you MUST have 93 or newer heads, and if the L67 gains 30hp from these rockers, the L27 isn't going to gain as much. For argument'* sake, say 15 or 20 horsepower (to be on the big side and be optomistic). Is that money worth the gain?
Or would you rather do a cam? The Aussies know the L27. They have cams for them. Try Crow Cams down under. Cheaper, but a little more work, but bigger gains too.