You probably haven't played many high-powered games then since 28 damage per attack is decent but only having four attacks isn't, but it's better than plenty of other things. Let's optimize it more shall we?
To give you an idea of what would be considered high-op for a rogue, it would start with TWF and crescent knives (which doubles attacks essentially), then add Martial Stance: Assassin's Stance for an extra 2d6 SA, Craven, and Unorthodox Flurry, which allows Flurry to work with a specific light weapon.
With the TWF feat and gloves of the balanced hand to grant improved TWF, that's 2 off-hand attacks and at least 3 main hand attacks, which are all doubled for 10 attacks total. 10 x (1d3 + 6d6 + 11) for a total of 10d3 + 60d6 + 110 for an average damage of 340 total.
Haste's extra attack would of course add another 2d3 + 12d6 + 22, and then there's a spell called Sakkratar's Triple Strike which works like haste but grants yet another attack and would up the average damage to 476 before counting the extra flaming property it bestows on your weapons for its duration.
476 damage in one round is pretty decent, but massive damage is one of the tamer things when it comes to D&D optimization.
http://www.minmaxboards.com/index.php?topic=1767.msg17675#msg17675 might give you some inspiration on how Invisible Fist can be used to even better potential.