I'm running a 5:1 4G on a SBP shift kit (using 5/8" shaft HS motor) and will add a few issues you might see:
1), the clutch drum. The front gear is pressed on the drum and subject to separation when you least expect it. I had to get mine welded after much headache.
2), 3 chains and slack happens. Not easy to keep all of them chains happy without resorting to an idler on both the longer chains. Lacking any idlers, when any of the chains are loose you can get "drivetrain wobble", like letting the clutch out too fast on a manual transmission vehicle. Wommm wommmmm wommmmmmm
3), the mount. It can twist/tweak easily under high power in low gears. 'Keep it tight!' one might say, but that's hard when you have to loosen 9 nuts/bolts for second chain adjustment only to find the adjustment may be off upon tightening due to mount tweak. An idler is practically a must-have for jackshaft-to-outer ring chain.
4), the 30T gear supplied with SBP kit is, IMHO, too small for 5:1 gearbox. I'm using a 36T and wish SBP had a 40T. Gear ratios vary of course, but if you have a 5:1 gearbox I'll bet you'll be happier with a 36T or higher. SBP made the 4-stroke shift kit with 3.7:1 Stage III in mind, so keep that in mind if you're using a 4G. Newer 4G may have diff ratio.