No, it's too thick. What game is this for?

Typically, 18 gauge wire is used for anything carrying current, and 20 or 22 for switches. I typically get spools of stranded wire from jameco. They carry various solid colors and gauges.

Just curious what would happen if he used the wire that is too thick? Would performance be decreases or something of the like?

There is a very slight increase in resistance, but that's negligible.

The major factor is the physical size of the wire and bundle of strands. They won't fit in the hole in the lug of the coil--strands will be left loose outside of the hole, which can be bad if they short on something else or break off and fall somewhere.

Also, if you're splicing two wires together, it's a little more difficult to get a secure joint since the sizes are mismatched.

18 AWG, stranded, tinned

Actually the resistance will drop and it would be super effective if it was not for everything else ForceFlow said that is right. The usability becomes a factor. If copper was infinitely cheap and people put connectors on wires and components it would be awesome.

Oops, you're right.

larger wire diameter = smaller gauge #, lower resistance, higher current capacity

Regardless speaker wire looks & feels tacky used for non speaker uses... but thats just me being OCD.... the thought of seeing it under a playfield just makes me itch.

Ya speaker wire has no place in pinball electronics. Looks terrible.