(Until Hippo, or somebody that actually knows what he's talking about responds, you can chew on this)
That's a very vague question. Too Vague.
The WB E-series can be 'adjusted' for exhaust flow, changing the reversion waves or back pressure, and can be dialed in to a particular application. That's desirable with some pickie EFI systems like the Delphi, or it can be adjusted for engine mods with a carbed Bike. I don't know how they sound (obviously the more disc', the larger the flow, and the louder the report).
Thunder Header does put out good dyno numbers from what I've seen.
General concensous is Thunder Header Thundercones slip-ons do not perform well with the Delphi FI. Something to do with the reversion waves, I think...
I have always liked the TH but on the Delphi bikes it just seems like pipes with more backpressure work better.
I think it also has something to do with how big the engine actually is, not only in cubic inches but also on cam.
On the Delphi bikes I prefer the E pipe / Supertrapp type of pipes, but on some big engines you seem to find good numbers with the ProPipe.
Could well be that some cams like some pipes.
I've read some of your other posts touting the disadvantage of Thunderslide kits. I don't think anything could work much better from a driveability standpoint. Nary a cough in 2500 miles! I just can't bring myself to take the Dremel to the fins of my SEs to allow fitment of a Mikuni. I tweeked the CV myself and like I said, it's spot on. No experience w/Mikuni tuning even though I did download and print the tuning manual from thier site just in case.
Can you point out any particular negatives regarding the Thunderslide? Throttle response is acceptable for a CV which the major consensus say is a little soft on the hit. Circuit transition is smooth and I've yet to run out of top end. Pulls hard to 110 anyway... I tend to get a little nervous at triple digit speeds.
Do you suggest going back to the original style slide and spring?
If you are happy run it, no big deal.
It is possible that the lighter slide might improve throttle response marginally, but the lighter spring and the larger vacuum port have a strong tendency to cause slide flutter.
These are not huge issues and they tend to get beat to death. Not worth it to mess with a thing you are happy with for a couple of horses.
All these opinions are based on the premise that they take place before the fact, once you paid for the parts there isn't much point to them.