To add some maths to what @T3P3Tony said above.
Assuming 1.8 degree motors and 20 tooth pulleys we have:
120,000 steps per second limit.
Full steps per rev @1.8 deg per step =200.
So using 16X micro-stepping we have (200x16) = 3200 micro steps per rev.
So dividing the pulse limit by the steps per rev we get (120,000/3200) = 37.5 revs/second
A 20 tooth pulley using 2mm pitch belts gives us (20x2) = 40mm of belt movement.
So maximum belt speed becomes (40 x 37.5) = 1500 mm/second.
But belt travel does not correspond to axis travel on a coreXY - I'm a little rusty here but I believe it's a factor of 1.4(ish) so max axis speed would be (1500/1,4) = 1,071 mm/sec.
So somewhere around 1,000 mm/sec axis speed but anything which increases the steps per mm will reduce this. e.g. 0.9 degree motors will halve it and 16 tooth pulleys instead of 20 will reduce it by about 20%. Worse case 0.9 degree motors and 16 tooth pulleys comes out at about 600/1.4 = 428mm/sec axis speed at 16x micro-stepping.
Edit. Going back to the OP and the question of gearing:
If it helps I'm throwing around about 1.5Kgs on each of my XY gantries without any gearing. I use 1,000mm/s^2 accelerations but can go higher and my default non-print move speed is 350mm/sec. So unless the motors are very small and/or the moving mass is more than 1.5Kgs, it is unlikely that gearing would be necessary or advantageous.