Cheese789-
If I understand correctly, your supplier is asking you to spec less moly in the castings he's supplying you? From an end user standpoint, if he's giving you Mo and not charging you for it, you're lucky. If he hasn't caught on to the surcharge mentality, he probably won't be in business very much longer, especially if he's making moly grades. In other words, I'm surprised you're not already paying for the increase in alloy costs.
In terms of what decreasing or eliminating the moly (due to cost) will do to the properties of the castings, the short answer is: it's there for a reason, don't change it.
For best results, please give us the alloy composition (or designation) and properties you require (hardness, yield strength, etc.). If it's a stainless (CF8 vs. CF8M) type casting, then you'll lose some corrosion resistance, and I'm sure EdStainless could go more into effects of moly on corrosion resistance. Since you mentioned a briquette press, I'm imagining it might be either a tool steel or some other hardenable Mo containing alloy, in which case decreasing moly content will have detrimental effects on hardenability, wear resistance, and may increase tendency for temper embrittlement (in thick section castings).
But like I said, for best results, we need more info.