Top shot?
The last time i fished a topwater tuna bite it didnt matter about a topshot or leader. At night they cannot see the spectra anyway, or at least they didnt that night. the problem with no topshot is no strecth and loss of plugs. I would use 60# floro as a leader anywhere from 5 feet to 10 yards. But as of personal expierence, when the topwater bite is ON it is ON. Many of the blackfins crushing the popper didnt even get hooked in the mouth but rather the side.
When you are diamond jigging i will share what was sucsessful for me last time out. Drop about 100 feet, then drop slowly and jig, it will get popped. Reel him up, you will miss a few but just drop it back slowly again and you will prob hook up again.
Good Luck

DONOT join spectra to floro with a swivel PLEASE. i am sure you know this though.

If fishing topwater, you dont need the topshot or leader for anything other than a little stretch. Braid picks up off the water with ease and will more than likely not be seen anyways. Some reels and even rods dont cast too well when a knot is added to the line and that knot has to pass through guides. I almost always use a leader but my rods allow for this and using a leader just makes me feel a little more confident! If using tops, I wouldnt see any reason to add a lot of topshot, other than for the reason that snagged mentioned!!! That braid can really cut!!!

why dont you join your leader and braid with a swivel???

I dont just because it is added drag in the water. There are numerous knots for joining spectra to floro. Without a swivel you will cast much further(no 6 feet of leader with a popper on it, only 1 or 2 feet hanging down or however you usually cast).

I hope this helps

BF will hit anything, YF are a little more picky. Don't count on YF not seeing braid.
A SPRO swivel can be used under some conditions, others not.

Thats the fun of a topwater bite. i just dont like casting with a long leader hanging down.