21" Ported Horn design idea

Was discussing it on another forum we bounced about a few ideas, and there was a suggestion that making it an offset horn would put less strain on the driver. Modeling it up and considering a few possible layouts it actually seems like it's a better design this way, easier to construct at the very least. While I've tuned it slightly higher and it doesn't drop down quite as low as the other design, just about touching 30Hz at -3db is more than enough for the work I do (live music mostly) and think it's better overall. 

Another discussion was about port area. Around 400cm2 as is in this design (two 6" pipes) is about as big as you can go before starting to think about having to fold things, but people suggested it was very small for a big capable 21" driver such as the 21DS115. I'd like to ideally see a bigger area too, but it doesn't seem all THAT small when you compare it to other commercially available designs. Many of Void's designs use a series of 3 or 4 smaller pipes that give about the same, if not slightly less, port area, and things like Martin Audio's various ported horn designs also use ports that around the same size give or take. 

BassBox Pro, the program I most often use for reflex speaker design work, seems to think it's ok, and thinks that at 1700Hz, it's RMS, it'll only be pushing air at around 21m/s in the ports. You need to push the power up to over 5000w before you begin to hit what it considers the danger limit of 34 m/s.

I'd be interested on hearing people's thoughts on the port size, but I suppose it's something that can only really be answered for definite with a prototype and some testing, which I might try and do over the summer if I get a chance. 

I'm also not massively precious about the design. While I don't have any actual plans or cutsheets drawn up on paper, I'd be happy to zip up the solidworks file as it is for anyone who has access to that software, and they can get all the panel sizes and routing details and could have a go at a prototype if they wanted. 

(design is shown compared to the Othorn tapped horn in grey)

Im No designer, but this is a design i could be tempted to have a go at. 

But when you mention port size and air velocity being on the edge, i do get some concerns.

But as you state, its getting a problem with 5kw of power. Thats alot of power!