> a 6 inch 4 ohm speaker rated at 65 watts
A small speaker with a large power rating, mass-market, is surely a LOW efficiency speaker.
(There's also hot PA system array midranges, but flubby home speakers are a million times more common.)
Fondle the paper. Is it thin and hard (and light) like typing paper? Or thick and fluffy (and heavy) like compressed paper towels?
To get LOUD you want a large speaker or a large amp (or both). "Large" amps are not physically large and no longer very expensive. When customers demand compact boxes and medium loudness, the designer goes with a small speaker and "large" amp to brute-force the insufficient cone area.
Also small speakers "sound" small. Your ear can tell the size of a sonic object; sometimes small speakers doing full-orchestra can trick the ear and "sound big", but a naked solo guitar makes the source size more obvious.
If you are broke, but happy, do it.
If you can spear a few dozen bux, get a GUITAR speaker nearer 10". Aside from being light and efficient, it has "zing" that you don't want in a hi-fi but DO want when making music with the fairly boring sound of bare steel string on solid axe.
If you want a MAXimum of sound for a minimum of Watts (as in tubes or battery-power), go with the biggest cone(s) you can carry. A Four-Ten stack is an excellent start. But a tight fit in the Civic.
I have a concept that guitar "should be" a Ten or a Twelve, about the size of the active part of an acoustic guitar's soundboard. (The edges don't vibrate much.) And electric guitarists want "MORE!" and should start with the killer Two-Twelve Twin. (OTOH there's kick-butt sound in one heavy-duty Twelve with a 300-Watt amp.)
> the efficiency of the cabinet
Cabinets don't have an efficiency.
Speakers and their cabinets/baffles "should" be integral, but are normally made by very different workshops, and speaker makers don't like to get involved in cabinets (which are often more looks than function).
Cone speakers "need" a baffle to support their lowest octaves. The box has little effect in the speaker's middle register, but a naked speaker is gut-less. A large baffle supports down to a certain bass limit, a clever tuned box can support even better to a certain note then sacrifice all lower notes.
> Guitar cabs are basically the worst cabinet design possible from the standpoint of real engineering
No, they are (many of them) an excellent compromise between efficiency, projection, and size, over the narrow band and desired response shape for guitar. This is a different problem than what T&S published on. Thiele's work is applicable, but too high-level for ordinary people, and focuses on the "flat" result. Small specifically targets extended lows in small boxes, rather than the rising response and LARGE cone area needed for loud portable guitar amplification. Don't let the experts buffalo you. The Fender Twin is a fine design for ballroom use. The Twin with two JBL D-120s is an astonishingly efficient air-whacker, 100Hz-1KHz. It sucks below 80Hz and the phasiness above 500Hz is very strong. An unfaithful hi-fi but a wonderful guitar voice.
