DME Relay 15A vs 30A
 

I just got a replacement DME relay for my 86. The replacement was 30A while the older relay is 15A. Are all of the newer relays 30A? Does the amperage make a big difference?

Heck Yes,

Placed the OEM spec one. the higher amperage may burn out more critical components like the ECU, a more costly part.

my 2 cents.

The higher amperage is getting anywhere near the ECU circuitry. That is the exact reason a relay is used to begin with. The higher amperage would be going to the fuel pump and injectors.

This is the rating for the maximum current that the relay contacts can handle so a higher number is no problem and is actually a good thing.

Actually - No, it makes no difference at all - a higher amperage rating on a switch device like the DME means the relay contacts "can" handle more current. The rating doesn't imply that 30Amps are actually being drawn through the relay. The current drawn through the DME by all the loads attached to it will stay the same, because the ECU and fuel pump are the same. So, it means they've actually beefed up the contacts on the relay, and it will likely be very beneficial since the relay contacts will probably last longer...
An analogy: it's like putting a beefy 1" plumbing valve on a 1/2" water pipe. It still only has 1/2" water pipes on either side, so you only get as much flow through the valve as the 1/2" pipes provide. But it works... the valve is just bigger, tougher, and therefore better ;-)