Heartrate Analysis?

I am intrigued with the relationship between grade and pace for the same heart rates. If your pace is directly linked to your heart rate and the grade you are running on, I should be already be able to run a sub 22 5k on my flat course at training pace. This is based on running a 7 min pace at 85% heart rate for a single mile on the flatter route and knowing that I can hold 85% for 4 miles of hills already with no problem. I may test this out next week to see if it holds to be true. If this concept holds true, could I already expect to be able to race a sub 21 5k if it were held on my flat route?