I practice really intensely on the 6 or so exercises that I do from speed mechanics but I dont know when I should leave all the old exercises behind and try some new ones.Is there a certain tempo I should be able to get to?
whatever temp you think you will need to be able to play at, then if you ever find you need to be able to play faster then go back and build your speed more.
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i have the exact same book, and i normally level it out around 160 for 16th notes and such. depends how good you are now, because i had to start everything at 80.
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I tend to do exercises consisting of riffs from my band's songs as we are fairly technical.
My picking goes from 16th notes at 180bpm up to 16th notes at 240bpm in places, mostly alternate and sweeping. It kills 2 birds with one stone because I can practice picking and also practice my own songs at the same time
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WEll how about 176-180 bpm? should I move on since Im there or stick with it?

Do you feel comfortable with that as a good speed for those techniques? If so, yes. If not, keep at it.
^ well, if you feel it's fast enough and it sounds good, and you can pick what you want to pick when you want to pick it, then you can move on to something entirely new. If not, keep practicing...

It's up to you!