Well. 100 bucks is a steal for a HX-40.
The hx40 will have a TRUE 4 inch exhaust outlet compared to your hx35's 3 inch outlet so a new 4" downpipe will have to be aquired (they sell them)
The hx35 has a 56mm compressor
the hx40 should have a 60mm compressor= more air/lower egt
Depending on what exhaust housing you have on your hx35 now, the hx40 will probably spool slower becuase the compressor is so much bigger.
Things to look for-
Count the blades on the compressor wheel. Just so you know what you have
did it come off a 5.9 cummins? it has to have the right flange to mount to exh. manifold
What size exhaust housing is on it? its stamped on the back, if it has a 18cm it will be slow spooling and it will not light too quick, A 16cm was also offered on the hx40 but was kinda
rare and is what you want for quick spooling and more fun
Set the wastegate at 40 psi, anything over that your dangering your headgasket and the turbo itself might
grenade in the high 40's and 50's PSI wise.
Hope this helped. BUY IT is all i can say. to answer your first question, yes you will see performance gains, and this turbo is good up to the 400 HP range and is capable of much more. The HX40 also has a sweeeeet sound as well. You have to figure in buying or just making a 4 inch downpipe to the $100 your getting the turbo for.