I mean,how fast are you going that 'under-steer' is a problem?A beefier Rear Sway Bar will help reduce understeer on the 10th gen. Upgrading only the rear sway bar, and not the front sway bar, may change handling characteristics.
The OEM specs for the 10th Gen Accord are:
Sport and Touring:
Front Sway Bar 27.2 mm x 5.0T
Rear Sway Bar 16.5 mm
LX and EX:
Front Sway Bar 26.5mm x 5.0T
Rear Sway Bar 16.0 mm
Slowing down will help under-steer for free not to mention keeping your insurance rates low.