this is not a hard bug but a possible balance issue.
namely T5 belts are not very useful, they are much more expensive then T4 but only slightly faster.
put another way, there are few cases when you need a more then 60/sec but less then 75/sec, usually if you are using that much you use enough that 2 T4 belts are required anyway.

my suggested fix would be to boost the T5 belt to 90/sec. In addition to being a nicer number to work with, this is also the same ratio up as from red belt to blue belt.

I also understand completely if you dont want to change it, this is not something I would consider a major issue.

The way it works at the moment is that it's a calculated multiple of the base tier. Changing any specific one of them would make this difficult.
Also, Belts are very funky things, they need to be an exact multiple of a coded speed, otherwise the graphic of the belt doesn't match the speed of the items moving on it, 1.875. This number is derived because the speed value is stored in a fixed place number, and divided into "Pixels" which there are 32 in a tile. Basically 60 (Ticks per second) / 32 (Pixels per tile) gives you this number. (the exact math is more complicated, but that's the simple way to remember it). This multiplies up to 7.5 as the first "Nice" fraction, and 15 as the first whole number. So I chose 15 as the base speed.

A speed of 90/sec SHOULD work, but that basically means that as you progress up the tiers it goes... 1, 2, 3, 4, 6, so T5 gets double the bonus.
Also of interest to note is that if the belt overhaul is on, the speeds go up in different steps than if it's off.

Starting with Grey, T0.
On: 1, 2, 3, 4, 5, 6.
Off: 0.5, 1, 2, 3, 4, 5.

Though... before I even consider a change.... Why are you saying that the top tier is much more expensive than the others? The recipes are all identical, A quantity of gears and a quantity of plates, and maybe a quantity of bearings, of that specific tier, plus the previous belt IF the option to use the previous belt tier is on (more accurately, if you didn't turn it off), So why do you qualify Nitinol to be a huge cost increase over Titanium?

Interesting to know background info, I was mostly treating 15 items per sec as a given from the base game

I did not know about belt overhaul, will likely turn that on.

I think part of my perception of Nitinol that I never got large scale Nitinol running, so I never had enough to use the high tier belt with impunity.

I do still stand by that after 60/sec increments of 15 are not enough relative to the previous amount. were I designing higher tier belts for any mod I would use 1,2,3,4,6,8(,12,16,24,32 ect). This way the ratio increase is much more constant (rather then diminishing).

I have done different things with different mods.
I've tried the double every time, it gets silly pretty fast.
I tried +50% per tier, that does actually work fairly well, depending on what it is. Most of the machines actually use a scaling system where the increase is slightly more each time.
But Linear is actually used in quite a few places, Steam temperature is another area.
But Belts with specific steps that work, it was an obvious place to go with Linear. The inserters are actually tuned to match too.

Fair enough. While I still disagree, I am not the mod maker. I am willing to accept that it is your mod to do what you wish. :)

PS, because I dont think I said yet. Thank you, to both you and KiwiHawk for creating ,maintaining and developing such a great mod.

update, turns out i cannot turn on belt overhual because the option is disabled by seablock

Correction: with Sea Block you can't disable belt overhaul. Sea Block enables it then hides the option.

The problem with removing options, is that they don't always force enable/disable something as intended, they only delete the option from the list, and the state is whatever was previously saved. (Unless the devs have fixed this.)

so by deleting the option, all the mod has done is... lock it into the previous configuration that now can't be changed.

This is why I'm very against mods that delete options.

Even if it does work how mod authors intend, they're going against the original mod's design, so I'm still against it.

There's now a way of forcing settings to a particular state:

forced_value - bool - Optional.
Only loaded if hidden = true. This forces the setting to be of this value. This can be useful for mod compatiblity.[1]
https://wiki.factorio.com/Tutorial:Mod_settings#bool-setting

well, seablock is not using this. my seablock is forced into the disabled state. gray belts are 7.5 /sec

Gray belts only exist if belt overhaul is enabled.

I dont know what is supposed to happen, but my current seablock game has 7.5 /sec grey belts with underground size of 2

Yes, that is correct for Sea Block. Yellow undergrounds will have size 4 and are 15 items/sec.

For Sea Block specific discussions, I'd encourage you to join us on the Sea Block discord! https://discord.gg/veAEAbGSaB