Max recommended memory voltage for P55 is 1.65v. I'm not sure how safe it is to run higher voltage memory (or the negative effect it might have on the integrated memory controller), but its not something I would recommend.
I dont think bumping up the voltage to 1.75 would do it any harm. It would just run alittle hotter.
Heres from the manual:
Before you change the settings for extra-high overvoltage ability, use the BIOS items
frst to adjust the desired DRAM performance. Ensure your system functions well
under the highest BIOS voltage settings before you change the setting of this switch
• According to Intel CPU spec, DIMMs voltage below 1.65V is recommended to protect the CPU.
• The system may need a better cooling system (for example, a water-cooling system) to work stably under high voltage settings.