Cash builds a good competitive engine, got nothing bad to say about them. To get those rpms you first got to get the kart free, low rpm drops does not necessarily mean the kart is free. I have seen karts losing 100 rpm or less get lapped. Work with your driver to instill patience, wheel control and smoothness. Make sure you are spot on with setup, tires and clutch (yes the clutch is huge) The Vortex single or double disc are the best for Jr's in my opinion. Hard hitting clutches cause to much hysteresis and as they tend to be either locked or unlocked can slow the engine with too much slippage. Whether thats right or wrong I don't care, it's my opinion. Yours may differ.
On the engine, I have never had a red plate pull past 6300 on my inertia dyno. I have a 500lbs wheel and they just can't move it. But that doesn't mean It won't do it on the track. A gear rule red plate is built differently than a non-gear rule red plate. With a gear rule you look for rpm, Without it you build for torque. You can have one or the other, but you can't have both, Again my opinion and you know what they say about opinions.