My 5DIV at 2500 ISO doesn't look very good either, unless I went high to ETTR then pushed back down the exposure in post. Personally I would back down the shutter speed before raising the ISO in your situation. I tend to drop down to 1/400 or even 1/320 to get more light when needed; I'd rather a little motion blur over higher ISO. If you are using a long lens and no monopod getting a monopod might make you look at your camera with new eyes...the blur could be coming from camera shake at the lower shutter speeds...not motion blur.
Ultimately I always ask myself...will my client notice the difference and is that difference big enough to warrant the expense of new equipment? The final resting place for 99% of my images is 960x1200 images on someone's instagram or 1920x1080 images on someone's website, if I'm the only one who doesn't like a picture I took then I did something right. For higher end clients such as magazines and when I know they will make prints, then yes I go the extra mile to get every ounce of quality that I can...but that type of customer is few and far between for me.