Actually we answered at the same time, you just hit the enter button first. (both at 10:25).
I should claify an ealier comment, Flex can be used effectively as long as the added friction is considered. Just remember turbulance is part of the friction and as far as I know, there is just about nothing worse than flex for turbulance. To clarify, I meant that the 90 inside corner of an angle boot is probably less that 0.5% of the friction in those runs and probably not more than the resistance of the grille itself leaving 99% of the friction as a result of using flex.