Some HPLC PEEK tubing connectors / male nuts / unions are rated up to 6000psi with finger tight nuts. 

The back pressure rating for tubing is dependent on flowrate, density of the solvent, temperature, and some other factors that make answers to this question infinite due to all the variables.

Another consideration is the burst strength of the PEEK tubing. It is important to keep in mind that PEEK parts will become less stable as the temperature they are being use at rises.