Bosses at the end of each floor should always upgrade both arm and armor.
1-10 any gear below +5 gets upgraded.
11-20 - Any gear below +10 gets upgraded
21-30 - Any gear below +15 gets upgraded
31-40 - Any gear below +20 gets upgraded
41-50 - Any gear below +30 gets upgraded
I have ran 1-50 6 times now. The last two 41-50 I went from 27/28 to 28/28. Then the next was 29/28. We opened 6 silver chests each 41-50 and 3 exploded, 2 didn't upgrade, and 1 did upgrade. Ridiculous.

Your current system makes it so if you are above +20 floors 1-30 are of no use to you. You gear will never upgrade. I have no issues getting the gear to +15. Its after that it becomes very grindy. So why boost those floors? Boost 31-50.