Connect Component RBG from your 8300 to Sling input (top row) then connect RBG from Sling Output (bottom row) to your monitor.
Do the same with your audio cables and you should be good to go.
BTW this is an issue with your cable box not the Slingbox. Your 8300 only allows one HD output at a time.
If HDMI is connected the Component outputs are disabled.
Did using componet cables work for you?
You are correct the component pass-through on the Slingbox is limited to 1080i.
Can you tell the difference is a good question.
When streaming content from the Slingbox to a PC, MAC or Mobile device the answer is no.
The stream is not even close to 1080i let alone 1080p.
Does the Slingbox pass-through do anything to the original signal other than pure pass-through? I don't know. I hope not.
Does Rogers provide 1080p content?
I took a very quick look at their website and it looks like 1080i max.
If I am wrong you could leave the HDMI cable connected to your monitor and plug it into your 8300 when viewing 1080p content from Rogers.
Kind of lame but it is an option.
Hope that helps.