It sounds like you trying to compare the secondary skills of 1 player to around 4? OFC the 5th bowler is going to lose that one. How do you feel about 1 5th bowler vs 1 slip vs 1 tail end bat?
I started off the thread with saying lower order batting, that can extend down to 9 for many teams and I never restricted it to no. 8, it's just diminishing returns much far down.
But we can do either.
We can compare the greatest cordons (1st through 3rd) vs the greatest tails (8 through 11) and take note of which had the greatest impact on their teams. We could include 5th option, plus support bowlers, but not sure if that comparison would be comparative.
We can also compare the best no 8's vs the best slip's, and again see which had the greatest impact on their teams and wins.
Even when we look at potentially the best or deepest tail, Australia 2002, I would suggest and argue that their slip cordons was still more important to team success. Think they would as well.
Even when we look at what's clearly the three best teams of my life time, the cordons for all 3 were not only superb, but the hallmarks and critical components of their success.
Warne / Waugh / Ponting
Lloyd / Richards /.Richardson
Smith / Kallis / deVilliers
The mid 70's Australian squad and the preceding SA team would be next in line and follows the same premise.
With regards to individuals, considering how critical taking 20 wickets is to actually winning tests, and how difficult they can be to come by, your second slip (who also transitions to 1st to the spinners) is at least as important if not more so than your no. 8.
I was watching some long form highlights of the west Indies in Australia in '84 and a partnership was building between Wood and another batsman where Richie made something out of what was at best a half chance. They also made reference to one that Lloyd had also taken the day prior. The average guys not only aren't taking that, there's going to be drops in-between.
In contrast I was reading some match reports from Pakistan, in I want to say Australia? where a key partnership saved the game for the opposition after they were dropped. And that shows up quite a bit for a couple teams over the past 50 years. It costs matches.
Lower order batsmen, and there have been about 7 of them who can be categorized as no. 7 / 8 batsmen, who can be seen as a tier above the rest, Dev, Procter, Vertori, Cairns, Imran, Pollock and Hadlee, are definitely important. So yes they can be impactful, but also incredibly inconsistent, hence the rpi in the mid to late 20's. And even beyond that, the wpm is also disproportionately low compared to the rpi, and that's because unless they're in abysmally bad teams that lose a lot, they generally don't get to bat twice very often. Hence again lowering their overall production and impact.
Setting a standard for 5th bowlers is tricky. Bowlers who predominantly bowled 3rd change or didn't match the production of higher slots, while maintaining a wpm of at least 1.5 leaves quite the short list. Greig has the highest wpm of exactly two, with 6 fifers and 2 ten wicket hauls, but none of the others comes close to that kind of production and of the four (the aforementioned plus de Grandhomme and McMillan) only Kallis and Greig had a s/r below 70. And there lies the rub, while doubtlessly vital and essential... For lower order batsmen even if not consistent, runs are still runs, while the wickets of the 5th bowlers comes generally at a greater cost and considerably slower. Of course that comes with the caveat that they never get the new ball and generally bowl at entrenched batsmen, but it's still a consideration. When they bowl, it's a drop off from the quality and production of your primary attack. de Grandhomme as was referenced earlier in the thread had a s/r of over 80, with one five wicket haul and no ten's.
So to answer the question, your elite slip fielder has the potential to consistently and efficiently impact and change more games than either. The tail lacks consistency and the 5th bowler lacks the efficiency.
Given the choice of Simpson, Taylor, Waugh, Sobers or Richardson in my cordon or Kallis, Greig, de Grandhomme, or McMillan bowling 3rd change in my lineup, or Imran, Vertori, Pollock, Cairns or Hadlee coming in at 7 or 8, I'll take the ones who will capitalize on all chances presented, if not creating half ones on their own.
We can all see this differently, but I'll take the one that has not only featured in and impacted victories more, but makes the bowlers, who again dictates victories, better.
A team can make by without a good 5th bowling option, they can even manage without higher end lower order batters, in the modern era however, teams haven't proven to be able to reach the standard of greatness (home and away) without the elite men in the cordon.