Are there any problems associated with under driving the alternator and power steering pump when using a Power-up Pulley Kit?

No, the ratios are only reduced by about 15% on the alternator, and 20% on the others. This means that you get full charge at 1000 RPM instead of 850 RPM which will only cause problems if you plan to drive your car at idle for long periods of time with the fan, high beams, air conditioning, rear demister and stereo full blast.

The reason for under-driving is that from the factory the accessories (alternator, A/C, power steering) are designed to run at almost 100% at or near idle, so at high RPM they are causing excessive and unnecessary drag. Reducing the drive ratio by 15% doesn't have a large impact at idle, but at say 6000 RPM, 15% becomes a much larger proportion and the reduction in drag is much more noticeable.