First off, this build will be as serious as my HP trolling build. This is only a casual build bred with theorycraft and nothing else.

A fact that few players don't know, is that Tiamat splash do stack. A fact that many players do not know, is that Tiamat splash stacks with criticals. Deceive, having a 100% critical rate, makes Shaco one of the best champions to stack Tiamats on. Also, Backstab stacks with this damage (need fact-check).

So how much damage does he really do? I did the (hopefully accurate) math.

No help:
Attack Damage: 452.7
Tiamats: +250
Shaco: +114
Infinity Edge: +80
Havoc : +5.7
Brute Force : +3

Crit Damage: +325.51%
Deceive: +120%
Critical: +100%
Infinity Edge: +50%
Runes: +45.51%
Lethality: +10%

Damage Multipliers:
Tiamats: 250%
Backstab: +20%

TOTAL DAMAGE: 5,778.85131

Maximum on Summoner's Rift:
Attack Damage: 680.7
Tiamats: +250
Shaco: +114
Infinity Edge: +80
Eye of the Storm: +50
Radiance: +45
Exalted with Baron Nashor: +40
Rally: +35
Raise Moral: +26
Hymn of Valor: +14
Elixir of Fortitude: +10
Aegis of the Legion: +8
Havoc : +5.7
Brute Force : +3

Crit Damage: +325.51%
Critical: +100%
Deceive: +120%
Infinity Edge: +50%
Lethality: +10%
Runes: +45.51%

Damage Multipliers:
Tiamats: 250%
Backstab: +20%

TOTAL DAMAGE: 8,689.33971

Maximum on Twisted Treeline:
Attack Damage: 600.7
Tiamats: +250
Shaco: +114
Infinity Edge: +80
Eye of the Storm: +50
Radiance: +45
Rally: +35
Elixir of Fortitude: +10
Aegis of the Legion: +8
Havoc : +5.7
Brute Force : +3

Crit Damage: +325.51%
Critical: +100%
[Deceive: +120%
Infinity Edge: +50%
Lethality: +10%
Runes: +45.51%

Damage Multipliers:
Tiamats: 250%
Backstab: +20%
Crest of Crushing Wrath: +18%

TOTAL DAMAGE: 9,048.3764378

Note: These damage calculations assume that you will land a splash hit with Deceive behind your target(s).