Evenflo Amp boosters work well with 4 points in my 10. I haven't tried a 5th point with them. It seams that the booster would redirect the 5th point enough to make it a liability--the booster works quite well to get the lap and shoulder belts oriented correctly.
RV-8a Built, Enjoyed, Sold
Two Kids: Built, Enjoying
RV-10, Bought, Rebuilt, Enjoying
Build / First Flight Video: http://www.youtube.com/watch?v=8f9HXzZT1dE
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