Found the fix. For whatever reason component video starts out at lo res, possibly by default You have to use the onscreen remote when you first access the slingbox (with the TV off) and go to the menu and then TV setup and change aspect/res to full 1080i and accept the changes. I guess maybe this needs to be set for all outputs from the cable box. It is already set for the HDMI cable and this will set it for the component video out to your slingbox. You will now have full HD even when the TV is off. And the quality is very good.
One thing, on my cable box, which is a Motorola VIP1200 it will reset to lo-res when the TV is used normally and you will have to reset again, but this can be done from anywhere.