I understand what you mean.
Take for example clusium, where you have like 6 or 7 gates with about 25k life between you and the wonder and so many fortresses with at least 25k life and units gets spanned constantly. But 90% pierce armor for buildings. Making the War Wagons not ever likely to be used in such a map.
The immortals are a strong line and can hit but are slow to be able to go in and destroy the barracks and econ buildings, meaning you need to use cavalry to try to be able to take out the buildings.
If you dont use Cavalry to take out buildings, you may not be able to get to the wonder in time cos the immortals tend to throw things as a wave and leave nothing behind but with gates that have 25k life and slow approach means then getting there in time is hard.
If speed was the issue then nobody would you Ballista, Onager, Catapult.
My point is try to make Xerxes allow the War Wagon to be or act somewhat like a Ballista, Onager or Catapult. then It would be used in a lot of builds with many units.
Ballista, Onagers, and Catapults are all used with a tanky frontline whose sole purpose is to soak damage so that those 3 can dish out damage.
Buildings only have .8 pierce armor, NOT .9 pierce armor.
But let's directly compare, yes?
If you decide to use Xerxes, you aren't going to use Immortals - You are going to use Sparabara's, which have low damage but are extremely tough.
With a maximum War Wagon build, they deal 312 ranged damage/second - This is 3x the damage output of the next highest small siege build, the Ballista, even taking into account the pop difference.
Now, against buildings, it's true - War Wagons deal significantly less damage vs buildings, sitting at 62.4 damage per second, whereas Ballista's deal 352 damage per second vs buildings, and absolute smash through walls due to their high splash damage (Which reminds me - When using any siege, if you need to break a wall, attack the shorter pieces - Splash damage effectively triples your damage to a wall if you attack those short stubby pieces)
But again - Turning War Wagons to Crush DPS is NOT the way to go - and 3x bonus damage to buildings is quite likely too much considering that War Wagons completely decimate armies at a much faster rate then the other siege.
I would suggest, instead, the following buff to Xerxes.
1 - In addition to seeing 20% farther, Xerxes increases the attack range of War Wagons by 20%. It's that way on pretty much every other Advisor that buffs line of sight - It also buffs Range by a corresponding amount.
2 - Xerxes also boosts War Wagon's damage output to buildings by, lets say 65% maximum.
3 - Alternatively, War Wagons ignore 30% armor.