Is it worth replacing this flex pipe with hardpipe
Flex pipe is the lazy way to put in ducting.
I think it should not be allowed for trunk lines and yes you were told correctly it is not as efficient because of the turbulence.
I am guessing this was a diy home because of the cobbled sections of round pipe to flex duct in the photo.
Unfortunately some use it more than others I only use it for short sections from the trunk to the registers.
Installing regular pipe will improve the flow the longer the flex replaced the bigger the improvement in flow you will see.