I can't comment on the Bren2, since I never owned one but I can comment on the SL8 since I did own one. In fact I waited for over 1.5 years before it became available and snagged one right away.
It's weird...I'm not a big guy and weight distribution is something for which I seem to be pretty sensitive, but I DID NOT find the SL8 (mine was a SL8-5, I think) front heavy. I would think I would notice this since I have noticed weight balance issues (ie. with my former PC Carbine with the aluminum handguard which I found very front heavy). The SL8 seemed to be fine.
I will say this about the SL8 - it was noticeably softer shooting with less noticeable felt recoil. I don't think it was just in my head - I really did notice this (compared to shooting my WK180 and X95). And the accuracy out of the box with just standard cheap PMC and Federal target FMJ ammo - it was a tack driver, compared to my other guns.
Some things I did not like about it:
1. some eyelet loop screw / bolt on the right hand side fell out while I was shooting and the front handguard thing fell off. I set it back, and tightened the bolt and nut and loctited it. I am shocked that that thing came loose when shooting - I did not expect this from HK.
2. This was my #1 beef as I am sure it is many other's number one beef - the magazines are just a dog. Double stacked in Canada, but blocked to 5, and no way to use my other standard Magpuls or LAR15s - apparently I am told there is a way with me either doing some dremel work in either the magwell or the magazines that I had. I decided to pass on that one.
For some reason the novelty on this thing wore off - could be because of the magazine thing (I bought and extra 5 rounder and it was over $100), and I sold it. Like the Kriss Vector, design and aesthetic oohs and aaaahs only last so long...