Aviafly:
I assume you are referring to the requirements in FAR 91.527. The language in paragraph (c) says:
Except for an airplane that has ice protection provisions that meet the requirements in section 34 of Special Federal Aviation Regulation No. 23, or those for transport category airplane type certification, no pilot may fly an airplane into known or forecast severe icing conditions.
The Beech 350 you refer to is, I assume, a variation of the Beech Model 300 (Super King Air). The Type Certificate Data Sheet for that model, under the certification basis, states that:
Compliance with ice protection has been demonstrated in accordance with FAR 25.1419 when ice protection equipment is installed in accordance
with the Equipment List.
FAR 25.1419 is the transport airplane category icing certification rule. So...in theory...it meets the requirements of 91.527 for flight into known or forecast severe icing.
Four caveats: First, what does the AFM or POH carried on board the aircraft say? That should reflect the TCDS, but it is controlling no matter what.
Two: Note that the TCDS language says "ice protection equipment is installed in accordance with the Equipment List". You'd better know what that is and be sure it is in compliance with the TCDS. For example, if you have an STC'd widget heater in place of the original widget heater, you're probably not in compliance with this language, and thus are not certificated for icing.
Three: Your avatar says you are in Europe. That may upset the applecart from the standpoint of operations; need to know more.
Four: Now that all that is squared away, we're back to PEI_3721's point: flight in severe icing, by definition, cannot be maintained. The only thing compliance with para (c) of 91.527 earns you is a get-out-of-jail free card when someone asks what you were doing in severe icing in the first place, assuming you land normally. Should you fail to land normally...many other questions will arise.
Hope that is of some value!