I take it you mean 8.4 2 hours after eating. The risks you want to take are entirely your choosing but here's some info.
It's reckoned damage can start to occur if you are regularly higher than 7.8.
19 out of 20 non diabetics will not exceed 6.5 2 hours after eating so 6.5 is where I aim for.
An average BS of 8.4 will give you an HbA1c of just about 7 so currently you are in the range that the NHS likes. Be aware that each 1% rise in HbA1c is thought to double your chance of diabetic complications. So at 7 you are twice as likely to have complications as someone at 6, at 8 you would be 4 times more likely than someone at 6, at 9 you would be 8 times more likely etc.
Many people on this forum aim for an HbA1c of less than 6 as at sub 6 you are nearly in the range of a non diabetic and all things being equal would run then run the same chance of complications as a non diabetic.
To get an HbA1c of 6 you will need to get your average fasting (before eating) and +2 hour after eating readings to around 7, to get an HbA1c of 5.5 you need average BS readings of about 6.
Remember an HbA1c is a measure of your average blood sugar level over the last 90 - 120 days although most of the value comes from what you've eaten in the last 30 days. So if you swap what your'e eating you will not see an immediate improvement in HbA1c even if you see improvements in your BS levels.
It is unlikely the NHS will encourage you to aim for an HbA1c of less that 6.5 as the NICE guidelines tell NHS staff not to put to much pressure on people but be aware that the NHS top of safe range HbA1c of 7.5 equates to an average BG level of 9.4 which is well above the BS 7.8 level where damage is thought to begin.
Hope all that helps.