As I mentioned earlier, I would try to upgrade my Hyakkaryolan with perfect stats if I ever got one.
Well I finally got it and I have started some testing with the upgrade stats.
I'm going to start trying to get perfect stats on mine even though I know it really doesn't make any significant difference.
I feel as though the reward for PS-Perfect with a perfect

50% deserves some extra time for perfect stats

I'm planning to watch a movie while re-rolling and re-loading and hopefully saving when I get max stats

As you'll see below, luckily for me, it seems it won't be too hard!
Code:
PP Att +PP +Att
------------------------------------------------------
Hyakkaryolan 360 952
1st try 369 959 +9 +7
378 967 +9 +8
387 975 +9 +8
+8~10 PP 397 984 +10 +9
+7~9 Att per upgrade 406 992 +9 +8
414 1001 +8 +9
422 1010 +8 +9
431 1018 +9 +8
440 1025 +9 +7
449 1032 +9 +7
2nd try 369 961 +9 +9
377 969 +8 +8
387 976 +10 +7
+8~10 PP 395 984 +8 +8
+7~9 Att per upgrade 403 991 +8 +7
413 1000 +10 +9
422 1008 +9 +8
432 1015 +10 +7
440 1022 +8 +7
448 1031 +8 +9
3rd try 369 961 +9 +9
378 968 +9 +7
387 975 +9 +7
+8~10 PP 397 983 +10 +8
+7~9 Att per upgrade 405 992 +8 +9
413 1001 +8 +9
422 1009 +9 +8
432 1018 +10 +9
442 1026 +10 +8
451 1033 +9 +7
The variance and range in bonus PP and Att. differs between each weapon type.
I know when I was upgrading my Robopitch Grenade I was seeing +17~20 for both PP and Att.
I'm going to conclude that the range for Twin Saber upgrades is 8, 9 or 10 PP and 7, 8 or 9 Att. per upgrade.
So the perfect maximum stats after 10 upgrades should be 460 PP and 1042 Att.!