Well, the question is one of capability versus actual use. Personally, if you can't arm a P-51D with 10 rockets, you shouldn't be able to arm it with 1,000lb bombs either. Or, if you can arm 1,000 lb bombs, we should be able to carry 10 rockets. P-51D POH (dated during WWII) shows the capability of 10 rockets. So, the capability existed during the war, even if they didn't use it.
I'd up Karnak's challenge to show a wartime P-51D in a combat zone carrying 1,000 lb bombs. To paraphrase the POH--those bombs created limit loads on the pylons and disrupted flight characteristics so severely that they weren't recommended for use. So, does anyone have photos of the Pony hanging crowd pleasers? I'd love to be proven wrong if you do so I can quit being such a grumpy pants about this issue.