Latency has very little to do with the speed of your computer. Latency is either the difference in time from the moment audio enters your computer, then exits your computer. Another measure of latency is the difference in time of you hitting a key on your midi keyboard, and the time it takes for your soft Synth(Kontakt, Vsampler, etc..)to deliver sound out of your computer. 53ms is acceptable when you are mixing audio in Sonar, but not if you are trying to play an instrument live. 10ms is said to be inaudible, but I've heard of people getting 5.8ms or better with the tascam you are using.
Latency is generally a factor of your sound cards driver. WDM has been said to work best with the Tascam USb, Windows, and Sonar. I've had better luck with ASIO drivers w/my soundcard. Your soundcards control panel has a parameter called buffer size. Next to driver type(wdm/ASIO) this is the most direct adjustment of latency. smaller buffer sizes will generate lower latency.
@ 58ms your problem shouldn't be pops and clicks, it ought to be an unreal amount of time between the time you press a key, till the time you hear sound. @2ms latency it is quite possible that you'll here pops and clicks. If you can get Sonar to register 2ms of latency, then your soundcard is doing its job, and you have low latency. If you are hearing pops and clicks, then your computer is not able to keep up with your soundcards latency. That buffer is being starved of data.
With your setup, you should be able to get 5-10ms of latency no problem, can your computer keep up with Kontakt streaming from disc??
What kinda performance do you get with the Edirol Soundcanvas? if you can get that to work without pops and clicks, then I'd imagine you could do well with Kontakt a 10ms.
one other caveat, Kontakt will run as a standalone, that means with out Sonar. Shut down every other program on your computer, including Power management, AntiVirus, Firewalls, everything. Disable your network adapter if you're worried about hackers. and run nothing but kontakt and your Bosendorfer sample. you should be able to run 2ms latency like that.