Think about a Hi Fi or Stereo as a complete System (recorded music studio and production, signal source (CD, vinyl), preamp, power amp, speakers, room).
No single device by itself can define the total results of the System.
The same goes for parts selection inside of a power amplifier.
Did you mean an 8 Ohm loudspeaker that is 8 Ohms everywhere from 20Hz to 20kHz?
Or,
Did you mean an 8 Ohm loudspeaker that has minimum impedance of 4 to 6 Ohms, and maximum impedance of 20 to 50 Ohms,
when measured across the 20Hz to 20kHz frequency range?
As to decisions of why to use a 2.5k primary, or a 5k primary,
Here are a few tradeoffs:
Primary DCR
Wire size
Number of turns
Maximum DC quiescent current before low frequencies will cause early core saturation.
Leakage inductance
Low frequency response
High frequency response
Required amount of laminations
Primary inductance
Primary distributed capacitance
Size and weight
Desired load impedance (related to the dissipation, etc. of the output tube).
. . . I am sure I forgot a few more factors
Why?
Design of an output transformer is an art, science, and exercise in economics.
Design of an amplifier is also an art, science, and exercise in economics.
Talk to the designers if you want accurate answers.
Engineering designs are a series of tradeoffs, sometimes they are not successful.
Watch the old B&W film about the Original Tacoma Narrows Bridge.