If the motor goes flat at 5200 in each gear, it’s very unlikely a fuel issue.

This would mean you could roll on the throttle in first, experience the flat line of power at 5200, and then right after the shift into 2nd, and the corresponding drop in rpm....... the motor recovers and pulls good back to 5200.

My experience has been this situation has always been an ignition or valvetrain related issue.
I suppose it “possible” it’s a collapsed muffler condition, but I can’t imagine that having the on/off switch-like affect you get from ignition or valvetrain problems.

Fuel supply problems usually manifest themselves as; the motor just starts laying down, and no recovery occurs until the throttle is closed........ fuel bowls start refilling, then you step back down....... car accelerates for a bit, then lays back down as the bowls start to empty.

With the XE cam, you’re certainly a candidate for a valvetrain issue.

Another thing, if it’s somethiing other than fuel, you should be able to run it hard through the gears(wot), and not experience any issues if you keep the rpms below the point where the motor lays down.
So, if yours lays down at 5200, you should be able to go thru all he gears at WOT and shift at say 5000, and not experience any issues.

If it’s fuel supply, you wouldn’t be able to do that.
You’d probably get about 1/3 of the way through 2nd and it would lay down.