I'd bump up the wire guage if you can,18 guage is still really small to get the power around your layout. A decent sized layout like yours needs larger wire to get the power out to the track, longer wire runs typically demand larger wire.
Lionel recommends 14 ga, I use 14 ga buss and feeders on my 2 10'x16' loops powered by one 180 brick. Each loop has only 4 pairs of feeders, I have an 8' passing siding and a few 6'yard tracks each of these has just one pair of feeders.
How are you checking the voltage? You should be checking it while there is a load at the track i.e with a train running.
14 watts out of a Lionel brick is low. It should be around 17.5 or so, unless you going through a TPC which will likely grab a watt dropping it to roughly 16.5
Your atlas loco may still slow down after you upgrade the wire, but in the long run bigger wire will be worth it for better power delivery across the layout.
