So really, you don't mean "reliability" in the strict sense.
All physical systems are a study in trade-offs. Back when the F-15 was new, the USAF decided it was time to try for a time-to-climb record. They took a plane that was about to be retired, and stripped it of all but essential components. Then they got some F100 engines that were close enough to write-off that they could dispose of them. Working carefully with the factory, the engine's "tune" was tweaked (sound familiar?) to gain more thrust. Then they took the plane and a small crew to North Dakota where the air is both cold and thin -- cold for better performance of the engine -- thin for faster flying. And they did obtain the record, having made the requisite two runs.
So what an engineer will tell you, is that pushing a boundary will result in a contraction elsewhere. In the F100's case, increasing the tune, shortened the time between major overhaul.
We see this in the aviation industry in general, and our own automobiles. As the flight hours, and miles (as the case may be) increase, we see an increase in the required maintenance both in frequency and extent.
So, if you're modifying the rocket to make it do more, then you're either eating into its overbuilt margins, and/or, you are changing the curve by which one should be performing required maintenance, usually referred to as "Mean time between _____".
When I was a kid I cut my teeth at an independent Mercedes shop. Sometimes, the doctors and lawyers who made up the bulk of the clientele would remark, "These cars sure are expensive to maintain." They were ignoring the consequences that they had chosen to purchase an automobile that in most respects ran rings around most American cars of the day - when they were in tune and kept up to date. But they were heavy, and ran through tires and brake pads. They had mechanical lifters back then, which required regular valve adjustments. They ran at higher RPM and were harder on their oil. And so on.
The practical aspects have been spoken here -- ask the folks who are running those mods, what they've had to do and how often they've had to do it, relative to the stock maintenance schedule.
If you're contemplating running something no one else has done, you would be well served to analyze what the contemplated changes would do to the stock maintenance schedule, and take really careful records that may reveal trends.
Or just say, "To heck with it -- I'm gonna have me some fun, and I'll cross those bridges when I get there."
Wish you the best.
