If youve been using test for 15 weeks now its highly doubtful that HCG is going to do anything for you at this point unless you come off the t, and you said your not willing to do this.
Your body already has its bases covered on test, I mean your giving it amounts surpassing the bodies own production quota, therefore the body, Hcg or not, is not going to be producing much at all on its own.
If you stopped the t, and let it clear out of your body for a few weeks, then took a month or two of hcg therapy with perhaps clomid or/and an anti-estrogen, then maybe you would start to rebound a bit quicker.
But the answer to your question is basicly this... as long as you are administering exogenous test, you wont be doing jack by taking hcg.
And even if you come off I bet it would take 2 months MINIMUM to even get back to normal range of t production by your body, even with the aid of the ancillary medication.