"Stretching" the rear triangle is, for example, putting a rear hub with 140 mm width into rear dropouts with 130 mm width; you're forcing the dropouts apart to accommodate the axle.
As DW said 35 or so mph with a BBSHD, 30 with a BBS02 (that's what mine achieved with a 46 tooth front chainring and 11 rear cog AIR). My BBS02 with a 52V, 6 a-h battery goes at least 20 miles off road since I don't use the motor about half the time. However, as he stated, there seem to be problems with the systems now**********
Since weight isn't a consideration, you could consider a direct drive rear hub system. A 1000w, 48V YESCOM kit (26") will go 33+ mph on a flat road with a 52V battery. I put together a system for an MTB for about $600 (motor kit $200; battery and charger <$400), but its range is only 12-15 miles.
BTW, my LeBS was selling 2017 Haibikes with a Yamaha mid-drive for $999 at the end of last year because Haibike had excess inventory.
Look at the tutorials on ebikesca for more information.