Why do you not want the boot manager to be involved?
Your suggestion of discoonecting the main drive during installation, is extremely clumsy and uneccessary. Most comments, in a thread such as this, are comparing 7 to an earlier OS. To put 7 on another hard disk, with possible different specs, would alter the comparisons, however slightly.
Users have short, or unobservant, memories. The only way to achieve a really accurate comparison, would be by installing 7 and your legacy OS on identical computers. This is not a choice for the average user. What is nearly always a choice, is an installation on a separate partition, with the dual boot.
It is also required, something being constantly overlooked when performance comparisons are discussed in forums, is that both Os's are clean, or are customised, as much as possible, in precisely the same way, and have the same software installed.
It is ludicrous, to compare a fresh install of 7, (possibly on another hard disk or enviroment) with a seasoned legacy OS, with fully installed software.