My opinion
-use grabs, forced block and roulette on wakeup for pressure
-utilize his strong long range poke and punish game
-step G to get free side/back grab/1B/3B on an overly agressive sophie
-learn her moves well to know exactly when u can step, and when u can duck (eg 66B+KAB u can duck the A for free WR B combo/ forced block/ stance roulette mix up.)
-Grab are dangerous against sophitia. Roulette is not very effective because sophitia verticals track a lot.
-Too dangerous, because of whiff punish. If you whiff with Siegfried at max range, she is actually in the best range to perform her halfbar combo.
-Siegfried's step sucks and stepG is less effective, Sophitia verticals tracks a lot => Fail.
-Useless against strong player.
Yes, this is 8-2, because there is no issue, execpt rape your oppenant's brain.
For Ivy, yes this is 7-3. IMAO the second worst MU for Siegfried.
I don't have real solution. Don't be afraid to use 3B a lot, and unsafe mix up, especially when she is in WP or CL. Take care of block punishement in SW. 22B and WSAA are your best friend in this MU. 66B, 6B and dash~G may be useful. SG pressure is not easy but possible, and can force Ivy to attack.
Eg. SCH cancel ~ 2A+B/1B. Basic and simple mixups, but devastating should your opponent guess wrong. Matchup charts do not consider this.
MU charts consider mixup. But do you play online ? this mixup will never works against a good player offline. No guess, only reflexe.
But if you have true advices against Ivy, i'm open. :-). How to deal with WP6B+K ?
For mitsu : at midrange Siegfried keep the advantage. Don't forget to punish him. (2KB, RLC A and mist kB by 1B. RLC B, RCL A+B and a:B by 3B, etc...)
For Amy : Same way. (2B+K by WS [B ] kB. 66A+B by K. 236B by a+k[A])
Voldo : Voldo is stepable, 88B for 80 dmg works well versus him.
Against BT Voldo, 3B/grab mix up is very unsane. Siegfried has the best backthrow of the game ! 68 + amazing wake up : here mix 3B and grab again, because voldo must wake in BT again.
By the way, 11B is very usefull against BT voldo.
Endly there is a lot of move to punish with a+k[A] : 3A, etc...