Run both of ours on a daily basis, stg 1 4x4 and stg 3 2wd.

800 miles a year... are you sure

Anyway, all cossies are reliable as long as you don't go silly power (even then they can be reliable), and maintain them properly.
I've had my 4x4 7 years almost to the day and used it daily without any major problems.

Now, the more power you get out of the engine won't necessarily decrease reliability, but it is likely to reduce overall engine life, meaning more frequent rebuilds, there are no absolute figures, so can't quote anything here to help.
My 4x4 for example has 120,000 on the clock on its original engine, I doubt a stage 3 would last that long.

Most important thing to bear in mind is that these girls are not getting any younger and by and large fall into classic car maintenance scenarios. Things are rusting and need replacing, old items like looms perish and need replacing, and that's potentially where hidden costs exists compared to a more modern car.
The car isn't unreliable like a new TVR, it's just unreliable-ish because it's getting old

Hope this helps.