I guess the relevant question is, does impulse have its own power source in addition to enjoying "warp power" whenever the latter is available?
Good point. According to "The Immunity Syndrome" Scotty added impulse power to the warp drive to create a bigger forward thrust.
The question thus should be, IMHO, if he used "normal" impulse power (fusion energy channelled through dilithium crystals?) or auxiliary/emergency impulse power (like in "Mudd's Women" or "The Doomsday Machine", possibly battery-powered laserfusion).
@ blssdwlf & Albertese
I'd like to turn your attention to the gridded frame structures on the TOS Enterprise's
"port", starboard and "bottom" engineering hull bow. This is essentially the same kind of structure seen on the "Phase II" Enterprise
nacelles (before she got her front "sinks") and on top of the sled of the Oberth Class.
If the protective screening of the main deflector adjusts near the engineering hull just to cover the extending parts interstellar matter could be taken in there, too (in addition to the ring segments behind the forward caps of the nacelles).
And an ion pod lowered from the bottom would instantly be able to take ion storm readings outside the shield protection of the main deflector.
(this is NOT an invitation to restart the ion pod location debate, just an idea).