Hey there DuaLoud,
to sum it up shortly, yes they do matter to some degree for certain Pokemon.
If we take as an example Weavile or Gengar. They all are both are generally played with 31IV's in speed, if you are playing ranked PVP in 9/10 cases the opponents Gengar/Weavile will be at maximum speed. If you have a Weavile with 30IVs in speed and your opponent has one with 31 IVs in speed, you will 100% every Speed-battle but if you both have the same IV's then it's a speed-tie (50% you win/lose).
I personally aim for +24 in all stats and maximum speed on all my offensive Pokemon, defensive Pokemon generally are good with +24 in everything.
As I said, it is more about speed-ties and outspeeding certain speed-tiers. :)
It should be if it has its Hidden Ability, has a Naive nature and has HP fire (not sure if you can have Hidden Power fire with an odd speed IV)
Dugtrio is banned, Blaze Blaziken is allowed, Speed Boost Blaziken is banned
Hope that clears some things up for you.
Kind regards,
Q8