It's a good idea, but it would collide with variable expansion. For instance, 10M is equivalent to 10*M where M is the value for the variable M.
Thus, you can get what you want by defining e.g. Gi=1024**3: Then 10Gi indeed means 10*1024**3. I thought a bit about pre-initializing the variables Ki, Mi, Gi or defining them as constant, but then I got doubts whether this is a good idea: Both would be rather confusing features for the user.
Moreover, there is really no need for k, M, or G: Since scientific notation can be used, you can just append e3, e6, or e9 to a decimal number instead, respectively.
Such a simple extension, yet so useful! Sure, there's Wolfram Alpha. Sure, there's even Google's Search evaluation of arithmetic expression. But /nothing/ I've tried beats the speed and simplicity of this little calculator extension. The workflow is unbeatable: new tab, type 'calc' (I saved an extension bookmark under that name), type the expression, return, done. Very little things I'd like to see changed, but maybe one feature that would be appreciated would be the ability to *copy* the returned values (right now, selecting the return line doesn't seem to work, so copy fails). Other than that: perfect!
(edit after response) Thanks for helping me out here. Guess my sloppy reading of the options lead me to miss that this extension's even better than I thought :)