Warning on Perpeturm from a race report on here. DO NOT mix it the day before
I actually went to a gastro for a year or more trying to figure out my stomach which was ultimately labeled IBS. The thing that really turned it around was running. Once I got into distance running and back into excercising 5+ days a week I hardly worry about my stomach at all anymore!
So far I have toast and a banana before things, then just water and Gu (Tri-Berry plavor) during the race. During the marathon I took a banana from people handing them out around mile 18 and took one water and one gatorade at each station in the later miles and did great.
I am training for my first tri though and am glad you posted this because was curious about trying some of the long bike ride type mixtures like Perpeteum and Heed.