There is no reason not to use the clan xl engine in all of them they will have a lot of tonnage for weapons and ammo, IS could do this too to a point ( low slot space due to 2x slots per upgrade ) if XL engines were a good option for most heavys and mediums but we know it's not for most of them , so many of those glass cannon IS builds will be not so glass cannon clan builds with more range.
As it stands IS will only have an advantage with ~50% quirks like the grid iron and thunderbolts and mostly in brawling range where large IS ballistics and short burn, cooler lasers and pulse lasers perform better , but we'r not exactly in a brawling meta right now, I guess we'll need to hope that PGI refreshes maps so they have at least one well covered rute to aproach, hopping from cover to cover.
Edited by Nik Reaper, 26 June 2015 - 12:21 PM.