I don't know a lot about S4 vs. S3, but plot reminds a lot of S3's on a KADE24 (other than the rev range, lol) - and the S3's on some old SCC cam-swap testing.
Cams like S3's seem to deliver about +10whp peak, mostly up. Back when SCC had my car they did a large amount of cam testing on the original motor (OEM, USDM vs JDM 10:1 I have now). Pretty serious gains but lots of JWT Tuning time, esp for the C2's. Dave C, Mike K, and Rob whose last name I don't recall were doing a lot of this testing:
IIRC the C3's are practically full-on race cams. I have a new set of C2's not yet installed, the ECU has a switchable maps for stock + 2 profiles for the C2's, and +37 whp peak is pretty impressive, but it's still easily $1000 investment for the hp, lots of $$$ there and the S3's/S4's look like a better choice for street-friendly. Also I like Calum's work, the RT ECU I have for my KA is nice work.
Sorry this pic isn't bigger, it was before I uploaded it, stupid photo gallery. Notice their dropoff well before 7100, too. Max gain came at 7100-7300 over stock, but max hp was back down around 6500 - a lot like the average I/H/E stock DE w/ECU.
Also keep in mind he was running earlier Hotshot header and 2.25" DP, not 2.5 or the larger 4:2:1 SSAC primaries:
Last pic the car wasn't running well, it was hot as hell, yet still pulled a 161 whp around the same rpm range.