Wouldn't hurt to contact the maker, but from my owner manual one would think they OK it by omission? No mention made in section 1 below 'bout not using +P in a Walther-only not to in certain S&W. Section 2 states no +P+ in Walthers.
So...+P is OK in a Walther (since it's not mentioned in section 1), or is it a misprint/accidentally omitted?
From page 12:
“Plus-P” (+P) ammunition generates pressures in excess of the pressures associated with standard ammunition. Such pressures may affect the wear characteristics or exceed the margin of safety built into some revolvers and could therefore be DANGEROUS.
This ammunition should not be used in Smith & Wesson medium (K frame) revolvers manufactured prior to 1958.
Such pre-1958 medium (K-frame) revolvers can be identified by the absence of a model number stamped inside the yoke cut of the frame. (i.e., the area of the frame exposed when the cylinder is in the open position.
“Plus-P-Plus (+P+) ammunition must not be used in Walther firearms. This marking on the ammunition designates that it exceeds established industry standards, but the designation does not represent defined pressure limits and therefore such ammunition may vary significantly as to the pressures generated.