"你有说什么就说吧" is not correct.
we often say 如果你有什么要说的，那（就）说吧！
this is how chinese express that english sentence.
of cause, you can say 如果你要说什么，就说吧！
you even can omit “如果”
just say "你要说什么，就说吧“
but,these two sentence is not that polite ...
"If you have something to say, then say it"
in this sentence,that person haven't say what he/she going to (or want to )say,right?
so, we use "要" to express this kind of meaning.(i don't know this word--meaning--is correct or not...)
in chinese when “要” as a verb,except "demand, ask, request" it also means "must, want, wish, desire, ask someone to do something"
as capriccioso says,if=如果
and we sometimes use 假如，倘若（often used in written language）,they are same..