[pdonis]

> I always understood that the Stress–energy tensor in general relativity has a momentum component, so two electrons whizzing past each other at near the speed of light would exert a stronger gravitational pull between them, in all frames of reference, than if the electrons were 'at rest', relative to each other?

The stress-energy tensor does have a momentum component, but remember that all components of a tensor are frame-dependent. So is "gravitational pull". Obviously the trajectories of two particles passing each other at relativistic speeds will be different from the trajectories of two particles at are at rest relative to each other at some instant; but the difference is not quite as simple as "more gravitational pull", although the two particles having relativistic velocities does mean that the center of mass energy of the system is larger than it would be if both particles started out at rest.

(Actually, the electromagnetic interaction between electrons is so much stronger than the gravitational that the gravitational effects are negligible in the scenario as you state it; but we could eliminate that issue by considering, say, two neutrons instead. My comments above assume that the scenario has been modified accordingly, which is why I said "particles" instead of "electrons".)