Unfortunately, I am very hesitant to change Plotter at this stage but there may be a workaround.
The custom PSP value is stored in the Windows Registry and you can try to set it there [HKEY_CURRENT_USER\Software\BTZS Software\Plotter Plus for Windows\Settings:PersonalSpeedPoint]
It is stored as a fixed two-decimal place value using a x100 multiplier (i.e. 2.4 is store as 240 decimal). Just make sure Plotter is closed when changing this or it will be overwritten when you open/close Plotter.
I make no guarantees that setting values here won't lead to bad analysis, program crashes, etc... It has been a long time so I'm not sure that there is not some internal reason for the range limit either but it may be worth a shot for your situation as it is easy to change it back.