Yeah - "Natural" testosterone boosters shouldn't have any really negative effects like this and the positive effects will be questionable in a lot of cases. There are a few good products out there, but they aren't steroid-like by any means.
I agree that P6 is at the very least, a super expensive option for what you're trying to achieve probably. There may be...I don't know...at least a dozen products that do the same thing and are half the price or less.
One thing that a lot of people aren't commenting on in this thread, because genital shrinkage is often associated with hormonal changes and we just tend to think that route - there is another COMMON cause of this type of shrinkage. Stim-d1ck. Yes, stimulants can often lead to contraction and shrinkage (and even difficulty in urination). This is not really "shrinkage" and is not permanent, but it could certainly contribute to someone thinking a product will cause shrinkage like a steroid...but this certainly is not the case.