How much longer is not easy to answer but if you are putting your amp in the boot and your battery is under the bonnet you will need to run a power cable and also you will need to run some RCA cables from empeg to amp as well from inside your dash then once you have the car in bits it will not take a huge amount of extra time.

Installing stereos is quite a mechanical process running all the cables.

On my car the RCAs run down the left side of the car under the carpet going back to a amp with a sub out the front channel speaker cables come up the right side of the car along the sill and then to the speakers at the door. Also on this side is the ethernet cable to the back where the wireless goes.
The rear speakers is a very short run from the amp up to the speakers at the rear.
The sub lives in the boot.

One top tip is think of what your ultimate goal/system is and run all the cables before hand for that end and whenever you do an upgrade the cables are waiting for you.

So to answer your question i would say yes upgrade your speaker cables.

Advantages are better sound ( in theory), also less messing around extending cables down to amps, no need to chop off plugs when you come to sell the car as trust me you'll sell the car before you tire of the empeg, mine is on it's second car that said the amp is on it's fifth car.