Since you're running at 5000 RPM, heat is definitely a concern. Re-packing the existing angular contact bearings with high-speed spindle grease could help, but keep in mind that the original bearings might not be rated for the increased speed and load from your upgrade. If they're not specifically designed for high-speed operation, they could still overheat or fail prematurely, even with the right grease.

If possible, check the bearing specifications to see if they're rated for 5000 RPM. If not, you might want to consider upgrading to high-speed angular contact bearings that are specifically designed for your new RPM range. This would give you more reliable performance and reduce the risk of overheating.