Yeah number for that sort of thing would be really tough.
I my memory serves me correct one of the main performance increases is in the object orriented realm of things. So if you make use of objects and such moving to php5 you'll see an increase in performance for code that make use of them.
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You won't experience a huge boost. I don't have the numbers, but I've used php4 / 5 and mysql 4 / 5. Didn't notice any kind of lesser load or increased / decreased performance. You'd benefit more if you installed memcache and used it in your php apps.
Numbers will depend on what kind of application you are planning to port or write. Just arbitrarily throwing numbers around is not going to be useful.
Any measurable increase in performance will be visible in large applications -- and only if you already have an established baseline to compare to (otherwise, how would you know what to attribute the perceived increase in performance?)
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