You don't give any much detail here as to dilution and temperature. I've only gently scratched at the edges of this personal exposure index business, but the Kodak sheet for Plus X suggests 3 1/2 minutes in HC110 Dilution B at 20ºC for a small tank, so I'd think 5 minutes could approach a stop of pushing? I've ended up going with HC110 1+63 to avoid those ultra short times when I use it -- too hard to time accurately.
Sound right? No. It sounds like you are going for higher contrast - "N+1" in AdamSpeak.
Five minute and under developing times with hand developing can be problematic: minor variations in agitation, pour in/out speeds, tank and reel type etc. etc. can make significant differences in the results.
Try something a bit slower and more foregiving for a developer: D-76 1:1 used 1-shot is a very stable process around which to base your tests.
I find when my "EI" doesn't agree with Kodak's it is because I am doing something wrong. Rather than change EI I change technique.
So many people ask about help with film speed. Unless you have the testing facilities of Kodak you really can't determine someone else's speed. There are simply too many variables and personal preferences involved.
Really, isn't this the purpose of the box speed. Film manufacturers spend a considerable amount of money to publish speeds for the average user with the average scene. If you are getting something wildly different then you are doing something wrong. I'm always suspicious when someone is adamant that Tri-X should be exposed at 64 or 1600.