Following SLSS: Don't obsess over the grains in the versitips. You can usually go up or down a size or two without an issue. Worry about length more. Definitely try before you cut.
Also keep in mind these are density compensated tips (unless I misremember). Not like level tips (T-8 etc.). If you cut from the front you are cutting the fastest sinking section. If you want to maintain sink rate and drop weight/length cut from the back.
He called them “versitips” but I’m pretty sure these are the same as Rio
replacement tips - I only have one scandi versitip line, but the tips that came with the line are Rio
replacement tips. He can correct me if I’m wrong, but I’m not aware of a numbering system for the weight of tips for any other Rio
So if we are talking Rio
replacement tips (7 and 8 weight are about 7.5 and 8.5 grains per foot respectively, etc.) then I alway tell people to think of them a level tips. Yes they have a couple of feet at the front that tapers a bit that Rio
itself says is to help a bit with turnover. It is pretty minor, not like a versileader at all for example, and you have to look very closely even to see it. So I always tell people to just regard them as level tips, since for practical purposes they are. Rest assured a level tip will hang in a straight line in a constant current. There is a mathematical treatment of this problem in the fly fishing research page that was linked above, as well as a rigorous derivation of why a line of constant density sinks faster the larger the diameter that troutless is referring to above. The Rio
replacement tips are compensated in a different way, namely so that all the tips in a given weight set will have the same grains per foot - as discussed above this number is a very important one for matching tips to lines.
Troutless brought up a very interesting point but I have no idea if the short fraction that is slightly tapered in the front is also density compensated in the usual sense, for the decreasing sink rate with radius. I suspect not but I could be wrong. At any rate the taper is very slight and only in the very front. I feel like you would have to be a pretty sensitive caster to detect the effect of that taper on the end. At any rate when I cut them back it is always from the front, thus removing the taper completely, and nail knot a piece of 30# maxima chameleon (relatively stiff) to the front end and put the new loop in that. I’ve don’t this with maybe 40 tips in the last few years, usually cutting 15 footers back to 12-13 feet. They all cast beautifully, but I’m guessing the ability to fine tune the gr/ft to match the line you get with the replacement tips is where the biggest payoff in turnover comes from, not the minor taper on those tips in the very front.
The reason why t8 is very close to the Rio
7wt s6 replacement tips is that those are 7.5 gr/ft - t8 is 8 gr/ft and its sink rate is about 6 ft/second. Both are for practical purposes basically level, and will sink in a straight line.