Good morning!
I realize that I made one mistake in my last post: I used shield HP percentage bonuses as an example because the direct results are easier to see. But the exact same math works for shield recharge rate/time even if it’s not quite so obvious. I waited to reply until I was sitting at home, in-game, with a Moa and five T2 shield rechargers to play with so I could fit them and quote the exact, in-game values to you.
0 rechargers: 3125 shield HP, 937.5s shield recharge time, 8HP/s reported* regen rate
1 recharger: 3125 shield HP, 796.88s shield recharge time, 9HP/s reported regen rate
2 rechargers: 3125 shield HP, 677.34s shield recharge time, 11HP/s reported regen rate
3 rechargers: 3125 shield HP, 575.74s shield recharge time, 13HP/s reported regen rate
4 rechargers: 3125 shield HP, 489.38s shield recharge time, 15HP/s reported regen rate
5 rechargers: 3125 shield HP, 415.97s shield recharge time, 18HP/s reported regen rate
Looking at the HP/s regen rate, it is clear that there is no diminishing returns. Adding your fifth recharger actually provides you with more HP/s than adding your first one. The absolute reduction in shield recharge time does decrease with each recharger applied, but that’s not an artificial reduction applied by EvE as a stacking penalty, that’s just the fact that each additional recharger is reducing a smaller number by 15%.
QED
CCP knew what they were doing when they kept these stats so low everywhere except for purger rigs; the rigs are limited to 2/3 per ship, so no stacking penalty was necessary there. If you want to see the kinds of levels you were proposing, you’d need to make shield recharge time stacking penalized to accommodate larger ships that can fit more than 2/3 of these modules. (Which, incidentally, I don’t think is an inherently bad idea, just not a needed one.)
*as reported by the in-game fitting window, with my skills which are maxed out.
EDITS: It’s early and I haven’t had my caffeine yet, LOL!