S. R. Faruqi:

For the world to be wholly brimful of the glory/appearance of the beloved is a great deal, because jalvah is that quality that belongs to water. Both of them don't remain in one place. The expression tamaashaa dekh qudrat kaa is also fine, because this phrase is usually spoken when something astonishing is shown.

For the world to be brimful of the glory/appearance of the beloved is not astonishing, but rather is a spiritual reality, and a majestic and magnificent thing. Here there is a suggestion that when you will have created a mystically-knowing gaze, then something will be visible to you compared to which the world's being brimful of the glory/appearance of the beloved will become merely something astonishing.

Another point is that this whole world itself is a spectacle of power/nature. But compared to the scene of brimfulness of the glory/appearance of the beloved, this spectacle is unreal/nothing. The real spectacle will be when you'll see the world brimful of the Lord's glory/appearance.

FWP:

SETS == EXCLAMATION; IDIOMS
MOTIFS == GAZE; JALVAH; TAMASHA
NAMES
TERMS == IHAM

SRF is right to emphasize the wonderfully exclamatory tamaashaa dekh qudrat kaa , which is a kind of petrified phrase used to express astonishment (like 'Will wonders never cease?!' in English). Here of course it works both literally as part of a sequence of instructions ('first create sight, then look at the at the spectacle of power/nature') and idiomatically ('first create sight, then-- wow! look at the spectacle of power/nature!'). In the latter, idiomatic case there's a kind of 'words fail me!' quality that suggests the inexpressibility trope.

The juxtaposition of yak-sar labaalab is also striking. Of course it means 'wholly brimful', but it's impossible not to realize that it also literally means 'one-head lip to lip'. However, the second line doesn't take up the body imagery, unless we count the idea of creating 'sight' with its implicit 'eyes'. Do the head and lips somehow vaguely anthropomorphize the beloved?

There's also a kind of informal 'iham'-like effect created by the jahaa;N . It's positioned exactly where it would make most sense as a relative pronoun ('where x, there y'), and the first time we read or hear the line, in terms of probability the relative-pronoun reading has to outrank the 'world' reading. Not until we get nearly to the end of the first line can we tell that we probably need to go back and think of a 'world'. (Even then, if the second line had turned out to begin with something like dil-e ((aashiq to ... , the relative-pronoun reading might still have turned out to be right.) Is this effect poetically significant? I don't know. But ihams are so interesting that I'm just compiling every stray bit of evidence.