The encumbrance penalty for tempering is -1 point for each 1% encumbered body weight. When enchanting, the penalty doubles to -2 pts per 1% encumbered body weight. There is no encumbrance penalty if you receive the message ...not encumbered enough to notice, regardless of how much you are carrying.
Ordinarily, the increased tempering penalty for special items is an intrinsic property of the required pre-temper potion and not attached to the item itself. The sigil staff appears to be an exception to this since it has unique properties but doesn't require a pre-temper potion. For example, an ayveneh potion has a pour penalty of -200 regardless of the item it is being poured on. Pouring ayveneh on a plain 0x dagger will have an increased pour difficulty of -200. Rather than requiring a special pre-temper potion the PTB have apparently added an arbitrary penalty to the sigil staff itself. This is the first time I've heard of this. Unless its a bug. If not then the item's tempering penalty is 50% higher than the pre-temper ayveneh and eoveneh potions.
The numbers.
If this was a plain +25 orase runestaff your modifiers with a dirtokh potion would be:
Skill: 302
Stats: 77
Dirtokh potion bonus: +25
Item enchant penalty: -25
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Total = 379 which matches your dechanter bonus
With the sigil staff your modifiers were 77. Therefore the sigil staff penalty is 302 (379 - 77). Is it just a coincidence that the sigil staff penalty exactly matches your MIU skill or is there possibly a bug when tempering these? You may want to check with a GM. Thankfully a failed pour isn't too penal since it is only one dirtokh dose.
Mark