Anybody thinks that mere 5 levels contribute to real power difference cannot possibly be an "elite player", as they obviously have no idea where their powers are coming from.

For anyone of that sort, they c*ready farm the top TI as well as pass the 120 exam, so the question is, then what? They get a boost of 1k-1500 BP, but it doesn't really help them accomplish anything else.

The reason that 85s have huge power gap and then get closed is because of the brothers TI. It grants a huge power boost starting at 32k, with all new golden tools with golden secondary stat. In a week or two of farming to complete the set, as well as reasonable elemental setup, you can gain 3k power from that alone easily, possibly more. This enables ninja exam progress, which gives further power increase in the form of equipment tier up, which is roughly 1k per tier, up to 2 tiers(100 and 120). Overall, the climb from 32k to 40k will be relatively fast, maybe around a month, compared to from 30k to 32 or 40k to 43, which also take roughly the same amount of time (new feature release not withstanding, cave gave everyone an easy boost)
This non-linear power growth is what makes causes the power differences, not level in and of itself.

w/o TI it's 1500.
w/ TI it depends. The thing is, red tool in and of itself doesn't have a lot more stat than yellow (see the difference in two gourd). Where the red tools shines, however, is in the secondary stats, those can be up to 25% higher, which, comparing max value of all gold tier vs all red tier, would have up to 2.5k power difference.
But that's not the actual growth we'll get out of a new TI.
For one, we CAN get all max red tier stat NOW, if we just farm enough red gourd. Not an easy task, but it is possible.
For another, we are unlikely to get max stat on either, so the raw margin per stat is smaller (btw, it's about 60 stat lines, each providing 40 extra power at max)
And finally, one of the preferred stat is elemental resistance (of your choice), and those actually might not get bigger, at least according to red gourd. This means fewer lines of stat getting a boost
Overall, even if we get full set of red tier tools, chances are the power growth there will be only 1k-1.5k.

Of course, it is fully possible that new red tools have BETTER secondary stat than red gourd, we won't know. But at the same time, it is also likely to be a mix of red and gold tool which makes the replacement process really long. Which means even if the new cap come with TI, it's 1500 NOW and then an extra 1500 over the next 2 month period (on top of other regular growth such as 8 gate, magatama etc that will grow w/ or w/o new cap)

Not nearly as much as you make it sounds to be.