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Qbittorrent & Deluge default "half open" at a small number of 8-20 or so, where as Utorrent puts defaults at a high 500.  Pardon while I might make some untrue accusations while I try to come to understanding on the issue. 

I would tend to think "half open" connections in many circumstances are not using much of any resources, other than taking up a slots in "connections per torrent".  This slot space usage can be compensated for by just setting the "connections per torrent" higher and then getting a feel to see what TRUE number you really connect to.  I tend to find to download quickly having high "half open" and to compensate with a high "connections per torrent" works best.  Now certainly once you approach your bandwidth limits more "connections per torrent" just adds needless overhead and you'll get the file slower.

Now I'm only do public torrenting and the majority of the time I'm only doing a single file, so I'm looking at this from that perspective.  Opinions?

I've responded to high half open connection limits before:
index.php/topic,3899.msg19857.html#msg19857
(first part of that post)

All told, being unfirewalled in qBitTorrent so you can receive incoming connections matter greatly more than a high half open connection limit.
Barring "exotic" UDP NAT hole-punching, firewalled peers will never reply to half open connection attempts to them. This is quite likely the majority of ip+port values attempted.
On private trackers where most everything on a torrent is seeds, your end may be retrying seed ip+port values at a furious rate to try to find a new peer that joined the torrent in the last 30 minutes. Problem is, by the time the tracker tells you of a new peer...it's likely already a seed too!

It becomes a matter of goals -- do you want to upload as quickly as possible to others or do you want to download as quickly as possible from others?
These aren't completely contradictory, but optimizing one tends to de-optimize the other.