If it was not designed for the speed 3 I would say don't install it.. because you have to install the MAF sensor, I'm not sure if the speed 3 has the barometric sensor as well just like the regular 3 does but that's not a problem if it does since it does not go inside the intake like the MAF does.
the diameter has to be for the speed3.
CEL means Check Engine Light. If something does go wrong you will get one. It's not necessarily bad, it might be that the MAF sensor got some moisture from rain or something but I would say if you do get one maybe autozone or one of those places might be able to read what the error code is on the speed 3... but I'm not sure if they have the updated info.
If you get the CEL right after the installation check all the cables, if everything seems to be in place and plugged in then there is something the ECU does not like. Disconnect the battery to reset the ECU and reconnect it. If you still have that CEL then just remove the whole thing and put the original airbox back on. (this applies if the CAI or SRI was not specifically designed for the speed 3) As far as I know only the Mazdaspeed CAI is available but it's waaaaayyy too expensive
My recommendation is to wait a couple of months until AEM, injen or those guys release their versions, then you know for sure it will work
Hope this helps