What sort of horsepower would be the limit of the sierra push in driveshafts

I was planning on putting around 150 bhp through them but was surprised at how slim they seemed

would I be better using the heavier but stronger lobro flanges and shafts

They will be fine - I've ran a V6 through tripods and billys first set of shafts were the same on his 150bhp+ XE

no problem, we have done 3000miles with 165bhp through push in shafts with no problems at all.