I put the STS baffles in my side pipes originally with the stock motor installed and got a notable improvement in power when compared to the stock 2 into 1 through a catalytic converter back to 2 through two mufflers stock installation.
I didn't drill the cap ends on the STS baffles at this time as I figured that there was plenty of restriction reduction for the stock motor.
The center section of the STS baffle has a 1 3/4" pipe down the middle with a cap on it.






STS says that you can drill the cap to change the sound of the baffle but not improve the performance. I found a thread here about 2007 where a guy was loosing about 50ish HP because he had not drilled or removed the center cap.
It stands to reason that at some point exhaust back pressure by just following the spiral design is going to impact the performance of an engine if the engine produces a sufficient volume of exhaust. So removing or modifying the center cap might be beneficial, to sound and performance.
Since my dyno last year I've been looking for ways to optimize the current rebuilt 350 producing approx 370 HP. Figured I would change the cap design to see what happens. I didn't want to just remove the cap and have the center wide open due to noise at cruise, and I didn't think that drilling the cap would have much effect on performance, probably just a sound difference.
So decided to remove the cap and weld on a modified cap. Really just a large thick washer with the center opened up to 1" diameter.







Yesterday it was 70* out So I took that oportunity to try out the new baffle openings.
The sound in deeper and throatier sounding. I have read where some people found the STS baffle to be somwhat tinny sounding. At times is was..maybe..just a little.
Now it has a throaty baffled exhaust sound that is subdued at low power applications and absolutely agressive and snarly at full throttle. Awesome sounding to my ear.
I can't say whether it is putting more power to the ground or not. It sure does sound mean though, almost big block like.
If you have the STS baffles and have not drilled the center cap I'd say give it a try. I'm very pleased with the results.

 

I thought mine sounded a little tinny with them capped too. Every time I pull the pipes, I drill them out a little more. I'm at 5/8" now and really like the way they sound. I'm planning an engine upgrade within the year and CR will increase a couple of points. I'm sure I'll be tempted to go for 1" then , but I may leave alone because I heard the incresed CR would definitely change the volume level. Glad to hear your feedback.

 

You heard right. I'm now at 9.9:1 and the noise level did increase from stock CR. I don't mind it even now but it may not be too wife friendly. I won't say anything and see if she even notices.