Introduction
TL;DR If you are evaluating everyone using the same framework (even if you are adjusting for income) it's unlikely that your less-payed will ever be competitive. This is not because they do not produce good work: It's social and psychological effects that have an enormous and compounding effect. Even worse, they erode the morale of your offshore staff.
Discrimination is a money problem
Picture this: You have worked for years only for the money to be spent on US employees. You could argue that these employees pay for themselves, but the fact of the matter is that the initial capital required for explosive growth is extremely critical.
The cumulative effect of having less money than a US employee makes your long-term prospects worse with each year. And that's the objective component.
The fact is that US employees are payed higher salaries from day 0, and can afford to be more generous with their time: That money pays for house cleaning, better hardware, better healthcare, and other amenities. Coupled with the better infrastructure the US provides, there is an inherent unfair advantage.
Does that mean that US employees are better in any fundamental way? No, they are just payed more.
The psychological effects of money differences are the nail in the coffin
The fact of the matter is that money buys status in direct and indirect ways. Being able to work less hours and exercise more, being able to afford networking events, being able to hold "luxury beliefs" and use more of their time pursuing higher-risk tasks.
After a time certain beliefs become entrenched and these "traits" aren't recognized as differences in money. In fact, it might be socially unacceptable to say that part out loud, whether it's in a dating or a job context - what a coincidence.
It is important to know this, that it's not a difference in moral character, and it's not a reflection of your abilities. Knowing fundamentally that it is a money problem makes it much easier to work daily to the utmost of your abilities without getting bogged down by the injustice.
Conclusion
Don't evaluate employees cross-jurisdiction on the same framework. It is not fair.