Well, one drawback is that (at least for my sim) money earned in tips via the subway does not count towards your income if you have the "band career" - it does, however, register as income if you have a "Make xxx simeoleons from tips" wishes.
As for the amount - yes, you earn with every instrument a lot more in the subway than by playing for tips out in the open. I am, however, not sure if this is intended or merely a side effect. When many people tip you at the same time you will always get a combined amount when you perform outside. I think the game handles the rabbithole subway in a way where you have combined amounts shown as single tips because of the action compression low detail mode of a rabbithole. When my game is lagging badly (e.g. if time machine is starting a backup) I usually only get occasional tips with astronomic proportions like 11,200 simeoleons - with just 75 or so performances.
So I think the tips are not actually higher in the subway but the game puts more people into them that all tip at the same time thus causing the higher tips.
