toe clips, you can deliver force to pedals, for a greater fraction, of the crank's revolution. Ie they stop your feet slipping off - so you can push down longer.
Modern pedals with cleats, pehaps longer still, and also on the upstroke.
Clipless offer smoother delivery of more power through pedal stroke - and the great opportunity for amusing clipstacks before you get used to them.
clip stacks using clipless pedals have absolutely nothing over the clip stack fun you can have strapped in with old skool cleats! :D
imagine laying on the ground, feet firmly attached to the bike, with you trying to lean over and undo the straps to release your feet.. fun for everyone.. who happens to be watching you :P
I remember seeing someone lying on his back kicking his bike in the air trying to get his cleats disconnected, while his companions stood around laughing.
It depends a lot on whether you use the old skool clips with cleats or not!
If you do use cleats, then the performance difference between clips and clipless is probably negligible. If you are using running shoes with old skool clips, then the performance difference will be much more pronounced.
2 of the biggest advantages of clipless over clips in my opinion are:
1) easier foot release, especially in the case of a crash
2) ability to have float in the pedal/shoe interface, so easier on the knees.
I ride with toe clips on my fixie for commuting and weekend ~50km rides - more than enough grip for up and down stroke pedalling. Wearing normal shoes is a bonus too.
The world's fastest still use toe-straps on the track. Some in conjunction with clipless, others with traditional cleats.
So as far as absolute performance goes, toe-straps win IMHO.
State level track sprinters crack 2200W+ @ peak...I don't think the AIS will share their watts/time graphs with us though, it'd give too much away! But it would be more, for longer...