My original sentence probably didn't come across well Mr. Mutt.
Many times, the follow on aircraft improve upon the original or correct something that was really annoying. In the case of say the GII to the GIIB it was a greatly improved wing. From the early to later GIII it was AC/DC to AC only. Both times, the airframe was greatly improved.
My point with the GIV is that there were not any bad habits that were rectified with the change from GIV to GIVSP, so the extra 3-4-5 Million US are perhaps not a necessary investment.
FR