Thanks for getting in touch - the padding you're looking for is set via the leftIndentForTextView property - this isn't currently exposed however if you add the following interface to your project:
@interface SDataGridTextInputCell (indent)
@property (nonatomic, assign) float leftIndentForTextView;
This should allow you to zero out the leftIndent property, which should remove all the space at the left.
Let me know if this does the trick!