Since there is still no update or a prospect of a solution by HubSpot I have managed to create a work around to get the Deal Amount reflect the Quote amount without Discount or VAT/TAX so it represents the right turnover value for sales forecasting or analysis. You will need a professional or enterprise subscribtion because it needs workflow automations to work. Also this solution will only work if your company uses the total discount on proposals and not the discount per line item.
In short, I have created multiple calculated deal properties + some numeric deal properties to reflect the needed data on deals that now only exists on the quote like: Quote Discount, Tax / VAT percentage by country of the deal, Tax / Vat amount, Net deal amount without tax.
With the new created deal properties I have created multiple workflows to represent the net quote value without VAT / TAX and also show the given discount on a deal in a new property (so you can see how much discount is given on all your closed deals).
If anyone is interested in making use of this solution. Please send me a message.