We also have just purchase a Hewlett Pacard LaserLet 5100.
Probably it would be a good idea if you contact Bentley at
support@bentley.com with your question. I sent the following message to them: (The more they see people haveing need for a specific driver the more I thinks they will try to have a workable driver that will do the job for so many of us)

Rcently I aquired a Hewlett Pacakard LaserJet 5100 to be able to plot 11x17 drawings. I am having a problem with the element symbology as it prints to thick  and the drawings come out
not very clear unless I uncheck the Line Weights selection in the View Attributres. This occurs even if I select printer.plt (the system printer) which the one I refer to here.

I also have tried to select a different pen table, and or ajust it, but have had no luck getting a clear plot. Is there a driver specifically for this HP device, or is there any suggestion you can give to be able to make clear 11x17 plots of the drawings with this printer.

Any suggestion will be greatly appreciated
Thanks very much for your attentin to this matter

Try changing your plot driver.

File>Print>Setup>Driver

Select HPxxxx something. Try your luck.

Ciao.