I see fuel pressure regulation issues if you are T-ing the return line back into the fuel pump feed. This IS a restriction and restrictions, even ones that could potentially flow both into and out of the tank will cause issues. Even if your pressure seems to be what it's supposed to be, I could potentially see a problem with reliable fuel supply due to the feed from the tank needing to flow both into and out of the tank continuously.

It's not difficult to add a return line to a sender unit. A drill, some hard line and someway to seal it up, which can be something like JBweld, or epoxy will create a trouble free system that you won't have to second guess.