Grats Yarr.
1) Highest upgrade available for weapons is at 254. Atm there is bug which exceeds 255 resulting in weapon becoming 1/0 upgrade slot.
2) Highest upgrade available for armors and pets is at 254 <- Source from Sakisan. (Anyone with an SS of one should post it here to prove its credibility; personally I've only 244^ armor and 232^ pets at most)
3) Maximum upgradeable stats you can get is 360 on weapons and pets; and 468 on armors [Including 30% bonus from mythical armors](Refer to point 4 on how to get past 360 stats with armor bonus)
4) Armors including bonus can only exceed 360 IF you unequip it to nullify the bonus. Therefore, highest stats available from upgrade is also 360 without bonus.(468 including 30% bonus)
5) Items with more than >360 stats can be found; BUT however, it cannot be further upgraded.
6) Items with all insanely positive stats and no negatives is POSSIBLE. But it is just very very very very very very very very rare. *Hacked gears look similar to them. So unless you hunted it yourself, DO NOT risk buying it from an AFK shop as it might result in a ban. <- source from fluffycalico.
7) Survival pets are obtained AFTER clearing wave 14.

Maximum boost a guardian pet can provide is being capped at 40. Anything higher than that atm have the same effect of a 40boost guardian.
9) Magical barricade does NEUTRALISE the element-resist of monster. Only if that monster came in contact/touched the barricade (Tested and proven. Monster element will neutralise as long as it touches the barricade even before attacking it) *Thanks Subnuclear & Anarial for pointing it out
10) Weapon size of a squire and monk does affect the melee range of reaching a monster.
11) Weapon size of an apprentice and huntress affects the range of projectiles being fired. (A mini gun which is longer in appearance will require the monster to be at certain range apart before you can hit it)
12) Apprentice staves with more than +1 charge bonus will tend to have higher DPS than those lower than 0 when using auto-fire.[This being said, the higher the charge bonus is, the higher the DPS for auto fire goes] (It is more advisable to click manually if you have a staff of more than +1 charge bonus as it deals higher DPS)
#1 and #2 - The 200+ upgrade items (commonly noted at 200^) only seem to come from wave 20+ of NM, probaby NM HC. I can barely handle wave 10 before I get insanely bored. I can't imagine every farming that shit.
Also, #12 is a pretty big deal for us apprentices. I wasted over 100m on a staff that had no charge bonus only to realize it did worse dps then the 30m staff it was replacing.