How do you know if a new diet is improving a food allergy?

Around September 2013, my 9-year-old kitty Tigger started overgrooming to the point whether he created multiple bald spots/lesions on his back, stomach, and paws.  Tigger's veterinarian immediately suspected a food allergy, and so Tigger has lived in an Elizabethan-style recovery collar for much of the past 5 months.  An intramuscular cortisone shot in December seemed to alleviate some of the excess grooming (though it also made him very hungry).  Back in November, I tried transitioning him from his old diet (Purina Pro Plan dry food) to Royal Canin venison and pea hypoallergenic dry food, but he refused to eat it.  Then in mid-December I gradually switched him from Purina Pro Plan to Hill's Z/D low sensitivity dry food, which he readily ate.  It's now been ~8 weeks since he started eating Hill's Z/D dry food, and ~6-7 weeks since he has eaten 100% Hill's Z/D dry food (I transitioned the new food in gradually over a period of 10-11 days).

Unfortunately, there are still some concerning things for me:

- Tigger's stools have been either soft in consistency, or diarrhea, ever since he started eating Hill's Z/D food.  However, he is not defecating any more frequently than normal (he defecates every day, or every other day), and he has always defecated in one of his litter boxes and doesn't seem to be in pain while defecating.  Upon my vet's recommendation, I tried to feed him a tablespoon of canned pumpkin each day to increase the bulk of his stools, but after the first day or two, he refused to eat the pumpkin.

- Now that Tigger has been free from his recovery collar for about a month now, he still seems to be grooming more than normal.  He repeatedly licks the same spot, and also bites at it.  However, he has not created any new lesions.

- In the last week or so, Tigger has been vomiting up hairballs or saliva-ish vomit about every other night.  I have given him Vaseline (which he hates) and made special effort to groom  him (with my furminator brush), but he is still doing it.  I think he must be swallowing more fur than normal with his excess grooming, and hence he's spitting up hairballs.

My vet told me that it often takes 8-10 weeks when trying out a new diet to see if it's alleviating the food allergy or not.  For those of you who've managed food allergies in your kitties, does it normally take more or less than 8 weeks on a new food to start seeing improvement?  Are your kitties still grooming excessively and having soft stools/hairballs ~8 weeks into a food trial?  I don't know if I should give Hill's Z/D food a few more weeks or try to find an alternate food now and gradually transition Tigger to that. 

It can take up to 3 months for a true food trial.  The soft stools could indicate he isn't tolerating the z/d well.  He could also have something called psychogenic alopecia which is where they overgroom due to psychological reasons.  My cat DeeJay does this, she has a bald belly and inner thighs.  The typical meds for it is amitryptylline daily which DeeJay can't have because she has mini seizures.  She throws up hairballs about once a week or more often sometimes.  Did you see this thread about chronic vomiting?  http://www.thecatsite.com/t/272539/...y-2-months-can-be-sign-of-small-bowel-disease  It has a lot of good information in it.