I agree with what ghostpilot said. People like options even if they dont need them.
A spring kit that would work with all Sterlings would be the sh!t. The foregrip/regulator would also be good on the end user & yall's part. Nearly every player today is so used to a front grip & being as one is not offered, that might be a turn off to some people. The extended rail I had for a short time took automag VAs & foregrips. Might be worth looking into other marker grips for that many more options.