Hi Karin,
Rehmannia Eight is more suited to an aging chilly cat.
There are two formulas you could start with that would help. First is San Ren Tang (Three Seeds Decoction). If respiratory congestion; or inflammation of other epithelial surfaces (such as the GI or bladder) is a feature of this cat's history; or it has Damp symptoms like dermatitis tendencies, a 'clumped' coat, or chronic eye discharge it may help. It is a Damp Heat formula, and certainly diabetes mellitus in cats is a Damp Heat condition, unless it is very advanced.
If the 'moisture' component is less pronounced, then Ding Chuang Tang (Arrest Wheezing Decoction) would be ideal if you could find it. It's more crucial for the uncontrolled and dyspneic asthmatic. The problem is it contains Ephedra (Ma Huang), so its availability may be limited. This formula won't help the diabetes, though, unlike San Ren Tang.
Diet change away from a pro-inflammatory (i.e. processed, carbohydrate-based) is crucial. Raw diets excel at eliminating diabetes in cats. I don't know how you feel about them, but they're worth a try. They are dramatically consistent at resolving diabetes in the majority of cats and would be greatly beneficial to the asthma as well.
Best of luck, whatever you try out.
Steve