The screw does most of the melting, but the barrel temperature should be maintaining the melt temperature.
How do I know if there is too much back pressure?
The quick method is to make a significant drop in the back pressure, such as reduce by half (ie. 1000 psi to 500 psi) and see if the melt temperature shows a significant drop (>10°F or >5°C). If so, it is likely too high.
The best method for this is to plot back pressure vs. melt temperature to see the trend. For example, you might see results such as 445°F @ 1500 psi, 427°F @ 1000 psi, 425°F @ 500 psi, 422°F @ 250 psi, 420° @ 0 psi. This would indicate that any back pressure 1000 psi or less is likely acceptable, though lower back pressures tend to provide better long-term processes.
If you need a higher melt temperature to disperse a colorant or fill the mold, you should add heat using barrel temperature. Using high back pressure is a very unreliable way to add heat to a polymer and will introduce inconsistency to your process.