...I was told that using Manley frames would mean extra honey per inch of super frame. Is this true? Or is it just to make uncapping easier?
There aren't any simple answers in beekeeping!
Manleys are wider spaced and go 10 to a National box (-- noted that you have WBC, but see later)
SN5's on the Hoffman spacing would go 11 to the same National box.
So, yes each frame ought to be fatter, and thus give more honey per frame than SN4/5 on hoffman.
They should give more 'regular' comb, more easily uncapped with less cappings 'wastage'.
But I note that ITLD (who has excellent cappings extraction resources) claims that the maximum total honey per box is obtained by using super frames on the same spacing as in the brood (thus 11 per box with a National), and carefully lined-up with the brood frames.
However, with more cappings to be dealt with, the same yield is unlikely to be accomplished by the hobbyist using the same spacing!
I recall hearing that WBC's take 9 Manleys to a super, though I haven't seen it for myself (vs 10 hoffmans).
I'd suggest you stick with the hoffmans that you have for this season, using them close packed on rails to get them drawn. Buying sides and bottoms isn't going to be much different from buying whole frames at standard prices.
Once drawn, you could move them to another box fitted with 9 (or even 8) slot castellations, and refill the rail-super with more foundation frames. Repeat as required, potentially adding castellations to the rail-box for the last supering of the season.
The other economic option, if you really fancy trying Manleys, would be to get a pack of seconds at the Spring Convention ... and sell on your SN5's.
