Porting TF2 to a newer engine would be a matter of re-using major existing game assets(models, animations, textures) in the new engine, then re-implementing stuff like lighting, particles, physics via the new engine. Additionally, most of the FPS optimisation that will come from switching to a new engine will very likely come primarily from the engine being able to utilise the GPU more efficiently for FPS-heavy stuff like particles, materials and lighting. The current models and textures in TF2 are actually reasonably well optimised, besides which, polygons are probably the least FPS-intensive assets in a modern game engine like Source 2.
Of course, the amount of work this would take, and whether or not they'd be willing to put in the effort I can't say.