That's a flowmaster muffler and it does crossflow, not as good as a true X pipe, but it does crossflow.
You could add an X pipe, but you would see no net gain from it.
best 5.7 power adders:
STOCK AIRBOX. you will NEVER outflow the stock airbox unless you are running forced induction. as well the oiled cone filters are notorious for ruining throttle bodies.
long tube headers with cat delete. (shorty's are okay but the stock downpipes are way too small) once you have a better flowing exhaust you will want an X pipe.
better camshaft.
a tune to tie it together. (the tune is the key that unlocks the potential from everything else.)
Many years ago I used to run a $2000 bounty for anyone that had a CAI that would show an increase in power. The challenge was simple. we would do two pulls with the stock airbox, and then we would do two pulls with a CAI. If the CAI made more power than the stock box I would pay (even if it was 1hp). other small rules applied, like both had to have a fresh filter, both sets of pulls had to be done fully warmed up and in closed loop mode, and the stock box had to be STOCK. no hidden baffles to restrict flow and cheat the test lol. we ran hundreds upon hundreds of dyno pulls a month. more often than not, if the power level wasn't exactly the same between pulls, it actually went down. The only time we ever saw gains was changing them out on forced induction cars, and even then the gains did not justify the cost. I never paid anyone that 2k.