I've had a few of these back in the day, 380/550, never the 750 though, and I had 3-1's on them and they were alway's Piper chromed exhausts.
To be honest they do absolutely nothing for top end, actually make it lsightly slower as they're restricting gas flow, but they did sound awesome. If your looking for top end you'll need to go for individual pipes, I fitted Gibson pipes to my RD350YPVS a couple of years ago as I couldn't get Allspeeds anymore and I don't think Piper have been around for donkey's years.
Here's a link to Gibon's pipes, I know they're not 3 into 1's but you should still get a good sound out of them and better performance
http://gibsonexhausts.co.uk/gib_examples.html
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Losi Eight T - Nova Rossi P21-7T- Hitec 5955's & Spektrum + TeknoRC Losi 8ET - Neu 1515-1Y - MGM 16025 & Hitec 5995's.
RC Heli's - Align T Rex 600 Nitro Sport - X-Cell Spectra G gasser.