Agreed. Boost is the answer.

The 1981-1982 shrouded port cam makes peak power at 5600 rpm. Since.you don't like to run at that high an rpm, going longer duration won't help.

torque and power? Sounds like a need for some boost pressure.

The cam core have is the one from the shroud port engine. Isn't that different from the regular port A14/A15?

GX came is for 6400 rpm use. If your A15 is a 1979-1980, the stock can makes peak power at 6000 rpm.

I would keep the 6000 rpm cam and rev the engine up.

4.11 or 4.3 gears would make a bigger torque difference than the cam

OK, stats:
1969 MG Midget (1600 lbs/726kg)
3.7:1 diff ratio, about 22.8" diameter (580mm)
A15 and 60A 5 speed
H89 head port matched to GX intake with two HIF44 SU carbs.
Some bowl blending in the ports
Tubular header, 2 inch (50.8mm) exhaust, I guess.
Target 9.5:1 CR, fresh engine with .01" (.25mm) overbore and balanced

Intended use: I drive this in the mountains and canyons of Southern California. I use third gear a lot. I am a mature fella, and my driving style is mostly Grand Touring, although I sometimes get ornery and toss the little British tart about. But mostly I use an RPM range of 2000 to 4500 while cruising about up in the twisties. So torque is important. I also need power for passing, and passing on these roads needs to be quick. Grab second and floor it!

I have a good used A15 cam core I can send out for regrind, probably to Delta in Washington state. I assume the baseline cam to be the GX for conversation.

Need more info?

Suggestions?