How would one possibly pass that? You can't uploop jump into the spikes to pass in, you can't take your forward momentum. You'd probably have to get on top, wouldn't you?
Here's Loops 101, then, for Sonic 2, 3, &K.
(http://img516.imageshack.us/img516/6323/loops101tf9.png)
The rows of ? are path swappers. When Sonic comes from either side, he is set to go forward/behind the other side of the loop (depends which side he comes from). The middle one switches his forward/backward placement as he passes thru (assuming he doesn't switch direction). Notice that if spikes were planted in the center of the loop, there'd still be two ?s hanging free. You'd need to loop jump at an angle that passes through them without spearing yourself.
If someone constructed it and pulled it off, then I'd say it's a worthy addition.