If you want to balance these special abilities due to concerns that they're a bit too powerful for a non-PrC, why not require a feat as a pre-req.
F'rex: Before a rogue can take HiPS, the character must have the feat Stealthy? Sure, Stealthy will actually make HiPS more powerful, but it's a feat the rogue can't take on improved critical, and it fits the character concept.
Likewise, Improved Critical doesn't seem like a bad pre-req for Death Attack. Sure, it means a straight rogue can't get Death Attack until 13th level, but that only serves to limit its availability.
--G