I don't want it to behave in any other what than what it did in real life.
In that, I am sure we all agree.
I am also sure nobody wants that more than HTC.
but it should significantly affect top level speed, like in the order of several tens of mph.
The problem is that intuitive estimates like that are meaningless without some form of data to support it. At best I think any data that could be found would be in the form of modified drag polars because I'm sure there would have at least been wind tunnel tests involved in that type of modification. It is very unlikely that anyone actually conducted tests for top speed with the dive recovery flaps deployed, so data for direct values probably does not exist. So even if data can be found it would probably require some analysis to interpret. I know that HTC can do the analysis, so if the data exists and can be found, I'm sure they would incorporate it if they haven't already done so.
Without any real data, all any of us can do is little more than speculation.
Badboy