Many roll down the exit at the floor.
I've tried it. 2 reasons imo.
1. Typical 340 Headers immediately turn down...so are we flowing with headers used or just guessing what it ends up at.
2. The flow number goes up.
It's like tricking the port into thinking it has more floor radius than it has ..and that bump on the roof might be the diff of how well the rolling the floor exit works or doesnt. Not saying exh flow numbers are as important as a quiet port.
I don't think I'd leave the exh stock after porting the intake side.
Stock flow yields a exh flow ratio of near 75% @195-200cfm intake and 145 cfm exhaust....I'd still try for that.
Polishing, good.
Radiused seat work, even better.
You'll find laying the ssr of the x/J exh port can immediately shift the peak into the .700 lift with little effort.
If you play with the roof kink...try only removing maybe .030 and aim 'with' the floor. Remember its expanding as its moving past the valve..expand too fast will slow the air, the opposite of what we want in an exh port.