We have also looked at the difference between the original TIP3P and the CHARMM implementation, and found the differences to be very small:
Mark P, Nilsson L. 2001. Structure and Dynamics of the TIP3P, SPC and SPC/E Water Models at 298K. J. Phys. Chem. A 105(43):9954-9960.
Mark P, Nilsson L. 2002. Structure and dynamics of liquid water with different long-range interaction truncation and temperature control methods in molecular dynamics simulations. J. Comp. Chem 23(13):1211-1219.

Lennart Nilsson
Karolinska Institutet
Stockholm, Sweden