I an doing webservice test with VST and confused about something

Can someone help me for determining serviceThrottling optimum numbers ?

What I want to do is decrease execution time of methods above 1000 users.

By default , it works fine with 1000 user and max connection 1000. But for 2000 user, if I not increase max connection, the test gives a lot of error. So I had to increase connection numbers, but this time response times increase so much.
For example from 5 seconds to 17 seconds.

So, I need to handle other parameters in the config file.

For example in the mdsn site, adviced is 16 * core

on another site, 16 * processor

Lets say that I give these numbers as adviced
but response time does not change..

When I increase calls and sessions again response time does not change

I am realy confused, can someone help me on this ?