if you are good with fixed beams, this sound like a plan! just have in mind using fixed weapons on a speed courier is much more difficult ... optimal mass multiplier makes it a highly responsive ship, and time on target gets down.
for the bug - i had it. it looks to me like there is a workaround: always have a modul and a second ships to transfer in place ... i have swapped core internals succesfully doing so. but sure - better to wait.
Finished upgrading it and briefly tested it. Not totally happy with some of the rolls, but I'll fine-tune it later. With the 8t tank and the 3A FSD it still does 687 m/s (711 with a 4t tank) - still isn't too shabby for a combat build.
It sure is twitchy but using the fixed MCs, I didn't have any problems. Really impressed with the shields - let an NPC Python shoot me with turreted beams for the ~40 secs it took me to take it down and it didn't even cost me a whole ring. As a matter of fact I'm so impressed I'll look into undusting my Viper for the same treatment.
I'd really recommend it. It's tremendous fun to fly and thanks for the help!