White Haven's relief mission to Yeltsin
First, before discussing any technical feasibility issues, Hamish's comments should be read in the context of the amount of risk the RMN can/should accept in an effort to save a single heavy cruiser/destroyer pair. Truman, as a member of Honor's squadron, ran a risk many (possibly even most) Manticoran officers would have considered unjustified. Go back and look at the passage in which she "forgets" to tell even Honor what she's planning on doing. Truman took that decision on her own authority because Honor would have been obligated by normal RMN policies and doctrine to refuse her permission to run that risk. As a captain of a detached unit, Truman had the authority to make that decision on her own hook as the captain of a Queen's ship and she made it without involving her squadron CO. In that same context, bear in mind also that when Hamish arrives with two full squadrons of RMN BCs, he'll also have full knowledge of what "Maccabeus" and his crew were planning to try. Under those circumstances (though I am aware that this was never specifically stated in the book) Thunder of God would quickly find herself blown out of space and the legitimate government would have been swiftly restored on Grayson... undoubtedly with enormous public support, following the planet-wide broadcast of the footage of Honor saving the Mayhew family from assassination. So the lives at risk here are those of the naval personnel aboard Honor's two surviving ships (assuming they haven't already been destroyed even before Alice reaches Manticore), what's left of the GSN (assuming it takes on what's left of the BC after Honor gets done with it), and anyone on the planet killed by bombardment from space (assuming Benjamin refuses to surrender to prevent that from happening). And also bear in mind that Hamish knows from Alice's report that Benjamin knows a relief force has been sent for and that the smart move is to play for time. In short, we have a situation in which there are perhaps 2,000-2,500 Manticoran lives at risk, followed by maybe another 4,000 or so GSN personnel, plus maybe lives on the planet if the Masadans unleash a planetary bombardment because of Benjamin's intransigence. These are not small concerns, but they have to be looked at in light of what Hamish would be risking if he took his ships to the same level in hyper as that used by Truman on her return passage from Grayson.
Bearing the above in mind, what you have in the passage you cited is Hamish saying that he can't justify taking sixteen battlecruisers, with old-style crews, representing -- what? somewhere in the vicinity of 40,000 people -- into that kind of high-risk maneuver in an attempt to save a single heavy cruiser and destroyer which might either (a) not be attacked at all even if he makes a normal passage or (b) already have been destroyed with all hands, in which case the speed of his transit is moot. And in terms of that statement, you should also note that he says "I'm afraid I can't justify taking two full squadrons of battlecruisers quite as high as you went." The operative word here is "quite." He never said that he wasn't going to take them into bands prohibited by SOP. In fact, he is, and precisely because of the risks I sketched out in my first paragraph above. But Alice has just finished telling him that she went too high, bounced off the wall no current hyper generator can transit, and damned near lost the ship, and he's saying he won't risk repeating that unpleasant experience with 16 BCs. "Not quite as high" is a clear indication that he does intend to cross the SOP threshold, however. In fact, the only limitation it implies is that he's not going to completely pull the governors on his ships and absolutely peak out the available hyper technology to the same extent she did. Also, you ought to note that he never says he couldn't do that because of any technical limitation which would make it impossible, only that he can't justify doing it. There's a big difference between those two positions.