Drawing at 300 dpi and zooming in (it doesn't have to be 200% exactly, that was just an example.) or drawing at 1200 dpi for no logical reason is counter productive. I've been working for over 10 years and no printer I've worked with has ever asked for a 1200 dpi image. I've done billboards and banners and never worked at a dpi larger than 600.
If you need to add more detail you don't need to work at 1200 dpi. For example: You are printing at 300 dpi with a print size of 8" x 10", which is 2400px by 3000px. If you want to get more detail in, then keep the dpi the same and increase the pixel size to let's say 3600px by 4500px and then reduce down to 8" x 10" when you are done.
I've looked at your work. It's very good and has an open graphic style. I'm not seeing a lot of teensy tiny detail, which is how I usually draw. These are at 300 dpi in Photoshop and ArtRage