UPDATE: I contacted Eric Smith with Valve, who has confirmed that they were made aware of this issue back in January and that a fix for Incinerator is on its way with the next update.

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The Incinerator Killstreak effect has a visual error causing it to appear much larger than intended when viewed at a distance. At sufficient distances, the effect can totally eclipse the wearer's head, making it hard to gauge where their head hitbox may be.

The problematic material is "particle_glow_05.vmt" and contains the shader parameter "$minsize .0125". This shader parameter forces it to never "shrink" below a certain threshold, meaning that a particle effect using this material will seemingly "grow" with distance.

I have created a modified version of this material which removes the $minsize parameter and adds the three following parameters in its place:
        "$maxsize .35"
        "$startfadesize .475"
        "$endfadesize .65"
These parameters are taken directly from "sc_softglow.vmt".

The new material is named "particle_glow_05_nominscale.vmt".

Inside the uploaded ZIP file is a particle file "class_fx__killstreak_t6.pcf", which contains solely the "killstreak_t6_..." particle effects, which were modified to point to the new material.