What are you doing to your poor car? First it was police style turns, now handbrake torture! A problem you might have is stretching the cable, rendering the handbrake ineffective. Also, I have never found the need to apply the handbrake in a fast turn. It is far more effective (and less scary) at slower speeds. There's a guy in our club who does autocross with one hand on the wheel and the other operates the handbrake. Not good form but he's fast.
On a side note, I've heard of a "mod" for the B13 handbrake that involves taking it apart and reassembling it with the springs repositioned to emulate a "fly off" style handbrake like those used in WRC, while still allowing the it to be used as a normal parking brake.
I used to work in a car rental place and I did the e-brake thing....ended up losing control....went right into a ditch at 40mph.
It is way too easy to lose control with this antic. (How did I get out of not getting fired? I told my boss a "small animal darted in front of me." He knew I was lying, but we had a good relationship and he was an old time car nut himself so he knew what the score was.)
I used to do it in a dirt field, actually it was the fair grounds parking lot. I stopped one night when I saw a cop turn on his lights and make a u-turn. Luckily by then i had kicked up so much dust, he didn't see which exit I took and I was at the opposite end of he lot. I parked my car at my friends garage. He lived across the street.
I don't think it helps turning much. At least not with me, but it is very fun. Especially when you're with someone your trying to impress. I was pretty good at it. But I did learn about weight transferring in my car by doing it. So in the end, I think it helped my overall driving.