Arduino Sample Rate Question

I have a Arduino Mega.

1. What is the maximum sample rate I can set in the Fixed-Step Size dialog box? 1.a when it is External Mode 1.b when it is NOT in External Mode

When I am in External Mode and observing the data I can do about 0.05 seconds and the data is reliable. 0.01 and it isn't even close. I'm guessing the Override feature will alert me to this. I know it is probably model size dependent. Is 20-50 Hz max I can expect to Sample?

Really what I want to do is collect the data using Simulink Code. It is convenient for me to save the data using External Mode right to the workspace. However I would like to get something close to 1ms sampling rate. Is that achievable? With External? If it not with External how can I then collect the data? Would I have to add a SD card shield? Probably. (oh boy)

Ultimately this is for a college digital lab. I would like to vary the sample rate to see the effect on the data. Primarily to effect on closed-loop control. However I need to reliably set the sample rate (for digital control theory)

Sorry for so many questions. Really what I want to know is how to set the sample rate using Simulink and Arduino and what the maximum is. (If I understand the help files Override detection will tell me when I'm not getting what I want....atleast that is good info.)

Thanks Everyone. Jason