Nakata77
Simply put, apart from ensuring wing tip clearance for aircraft lining up, the issue of obstacles is really only relevant in respect of protecting landing aircraft from them. SOU's runway 20 is a Code 3C Precision Category 1 instrument runway. If you are in to complex puzzles try reading CAP 168 references to this runway Code requirements. CAP 168 is of course available FOC on the CAA website.
As the threshold (by definition the earliest point of for landing an aircraft) isn't going to change on 20, other than a blast fence/bank mentioned below, no additional obstacle issues are relevant to the starter strip.
The question of jet blast (bearing in mind the rail yard next door), and any mitigation of it is a safety management issue which the airport will need to resolve. If deemed necessary, any blast fence/bank is going to need to be of such height that it doesn't infringe the complex obstacle free surfaces needed to protect landing aircraft. I would speculate that its height will need to be around 3 metres or less. As you speculate, addition of 170 metres could prove a little ambitious if the jet blast issue cannot be resolved with a relatively low fence/bank.
Hopefully all the operational issues have been looked at in detail before the Master Plan was published, if not then a number of management staff will again have egg on their faces, something that used to be an all together too frequent occurence with previous management regimes. Rather than me bang on about them all again, if you've got time on your hands you will see them mentioned in many of my posts over the last 6 + years.