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prot0typ1cal
I'd fix your belt routing first. Go ahead and rapid through your XY limits. Let us know how that works.

Regarding bigger steppers,
Smaller steppers won't effect overall print speed by lowering the accel/decel rates, as much as having to limit print speeds because the belts are out of plane and twisted. Wacked belt routing is also going to limit your available/reliable print volume.

Let me explain. If you can accelerate slowly to 60-100mm/sec normal print speeds, your total print time will still be less than trying to accelerate fast to only 20-30mm/sec for the entire print.

Yea, it'd be nice to accelerate quickly, and print super fast. However, until you make a new carriage that clamps your belts so they are in plane, and straight out, don't spend money on larger steppers just yet.

Just my 2 cents.

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the_digital_dentist
In the second and third images, the belt does not appear to be parallel to the X axis. That is going to cause distorted prints.

In the fourth image, the motor looks much too small to be driving any but a tiny coreXY mechanism. That is probably why your motors are slipping.

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the_digital_dentist
Jerk is the allowable speed for "instantaneous" direction change. When the extruder is coming into a corner, the speed is decelerating. It will decelerate to the "jerk" speed and then try to change direction without coming to a complete stop. It's relying on the springiness of the belt and the motor torque to slam the extruder carriage around the corner. That is when layer shifting occurs if the jerk is set too high.

If you set jerk to zero, the extruder will come to a complete stop at every direction change and that will slow down printing. Curves are made up up short, straight segments and the extruder will crawl around curves. Jerk helps keep things moving along smoothly and quickly. You know it's is set too high when the motors start to skip steps which causes layer shifting.

Setting acceleration too high can cause layer shifting, as can setting speed too high. If the motor current is set too low or too high you can also have layer shifting.

See: [github.com]

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the_digital_dentist
The width of the belt doesn't change the pulley diameter recommendation.

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the_digital_dentist
To the OP: if the belt is physically slipping it is not tight enough and the cure is obvious. It is far more likely that the motor is slipping due to a combo of excessively high acceleration/jerk/speed and/or too high or too low current. What are the settings you're using? Photos of the machine might help.

Prot0typ1cal: 40 teeth is the recommended (by SDP/SI) minimum equivalent diameter (about 25 mm) for a smooth pulley with belt teeth riding against it, not the drive pulley diameter. 16 and 20 tooth drive pulleys are common and fine with GT2 belt assuming the typical 180 degree wrap in a CoreXY printer. Using larger drive pulleys increases drive speed, reduces resolution, and reduces available force to move the mechanism. The width of the belt doesn't change the pulley diameter recommendation.