If it's a gain on bottom end you want, you could put on a more restricting slip-on than the end cap and stock muffler[/b]
Not necessarily true.
There's a lot of misconceptions out there on "backpressure", restriction, and the true issue, velocity.
Any slip on should show a nice gain down low, even versus the HRC cap. Why?
To answer that question, we must understand why "backpressure" is not good for low end as commonly believed.
Most confuse the attributes of velocity with backpressure. Backpressure kills power period.
The reason this is confused is because when comparing pipes of similar design, ones that are setup to optimize low end power create backpressure on the top end. Small diameter pipes keep low RPM exhaust velocity high, creating a scavengening effect that draws more spent exhaust gasses out of the cylinder and more fresh mixture in, but when the exhaust volume raises, the pipe may be too small to flow well, producing backpressure on the top end. People then associate "backpressure" with bottom end power.
Now lets say you have a nice open pipe that makes good power across the board. Throw a stock silencer on there that makes the exhaust take two or more 180 deg turns - which causes backpressure at all RPMs - guess what happens to velocity, it goes down the toilet -- everywhere.
This is especially critical on an engine with a fairly "wild" cam - probably mostly so on engines with lots of overlap that need all the help they can get drawing in a fresh charge at lower RPMs. I've dealt with some 400EX's that wouldn't even run with any type of stock silencer - even a gutted one that flowed every bit as well or better than a 450R HRC setup.
So.... In this case, it's very likely that no matter what slip on you put in place of the stock pipe, you will likely realize an increase in bottom end power, while at the same time, increasing top end power. Especially if you're not running a stock cam.
The HRC end cap, while good still leaves room for improvement -- it's passages (the stock silencer) are still relatively small and there is no direct path out for the exhaust, it must still make it's way thru at least two chambers (I can't remember exactly).
In my past experience, the difference may not be huge- there will likely be a difference. It's also my experience that changing the header will not make a huge difference either. All in all, with a 4 stroke, unless the pipe is ridiculously restrictive or the engine just refuses to run, there are only relatively small gains to be had with changing a pipe - especially if you're used to 2 strokes.