Before I rebuilt my tranny you had to be at a dead stop to shift into 1st. The original owner of my car had taught both his daughters how to drive in it.
I can now shift into 1st going 25mph.
If you have a good syncro and if you have fresh shift linkage bushings, the Rennshifter will make it shift even better. However, you will still need to heel/toe to bump your rpms for 1st gear.
James offers two grades of spring stiffnes. I use both. I use the softer ones for gear 4 & 5, and the stiffer ones for 1st & reverse. This helps me to avoid accidental shifts.
But I haven't shifted into 1st gear on an AX course in two years since I attended an event in Tulsa. The PCA there has really tight ass courses. Some people like them. I'm not one of those people.
I always shift into 3rd at our events and sometimes 4th as well. Our SCCA group likes fast and open courses. I am running 225/50 15's.
What JP said makes sense. When I rebuilt my tranny, I picked up an extra 5th gear set and flipped it in place of 3rd. That is a sweet AX combo (but it will suck for track use because it creates a really wide gap between 3rd & 4th).
I think even with your larger tire, that's going to be one hellavu tight turn to require shifting into 1st. My engine has a decent low end. It produces 116 lbs of torque at 2300rpm to 5300rpm. I would almost have to be at a dead stop to elect to shift back into 1st.
However, if ya gotta do it, ya gotta do it! I have my shift rod biased for the easiest shift to be upper gears, and it just barely shifts into 1st. You could adjust your shift rod linkage (at the shifter lever in the cockpit), just the opposite. That would definitely help you.
Good luck with it!