10 gage is not overkill. You're house will not catch fire either by surges or because #10 wire happens to be capable of carrying extra current - the circuit breakers and the grounding system protects against that.
With respect to surges: surges are a temporary overvoltage or overcurrent condition which can cause thousands of amperes to flow. Your circuit breakers are rated for 10,000 amps of short circuit capability - which gives adequate protection for lightning strike surges and arc faults. However, a lot of capacitive energy can sneak through - therefore the purpose of the surge protector. Also, heavier gage wire has a higher fault current capability. For example, #10 wire can clear an 8,500 amp short in only 1/2 of a 60Hz cycle and withstand temparture spikes up to 250 celsius.. a #12 wire can withstand 5,000 amps. That's why a circuit breaker is rated to clear 10,000 amp shorts - a safety factor of two. So, technically, #10 wire offers better protection if by chance your circuit breaker doesn't clear the fault or a lightning surge current misses the ground rod. Now you know more than your electrician.
As for lengths of runs: a longer run will cause a higher voltage drop. Generally, you want to keep the voltage drop to 2.5% or less at your outlet. For a #12 wire, keep the length under 30 feet and voltage drop will never be a problem at max current draw of 20 amps. For #10 wire, you can run up to 60 feet before you have to consider voltage drop. Also, power supplies can operate at spec down to 114 volts (5% voltage drop) so you can double the above lengths and still be okay.
There is another "real" reason to use fatter wire: sound. My monoblock amps sound WAY better when I rewired with 10 wire. #10 wire has less inductance/capacitance to the instantaneous current demand called by the amplifiers.
With respect to surges: surges are a temporary overvoltage or overcurrent condition which can cause thousands of amperes to flow. Your circuit breakers are rated for 10,000 amps of short circuit capability - which gives adequate protection for lightning strike surges and arc faults. However, a lot of capacitive energy can sneak through - therefore the purpose of the surge protector. Also, heavier gage wire has a higher fault current capability. For example, #10 wire can clear an 8,500 amp short in only 1/2 of a 60Hz cycle and withstand temparture spikes up to 250 celsius.. a #12 wire can withstand 5,000 amps. That's why a circuit breaker is rated to clear 10,000 amp shorts - a safety factor of two. So, technically, #10 wire offers better protection if by chance your circuit breaker doesn't clear the fault or a lightning surge current misses the ground rod. Now you know more than your electrician.
As for lengths of runs: a longer run will cause a higher voltage drop. Generally, you want to keep the voltage drop to 2.5% or less at your outlet. For a #12 wire, keep the length under 30 feet and voltage drop will never be a problem at max current draw of 20 amps. For #10 wire, you can run up to 60 feet before you have to consider voltage drop. Also, power supplies can operate at spec down to 114 volts (5% voltage drop) so you can double the above lengths and still be okay.
There is another "real" reason to use fatter wire: sound. My monoblock amps sound WAY better when I rewired with 10 wire. #10 wire has less inductance/capacitance to the instantaneous current demand called by the amplifiers.