SPI bus connections - to loop or not to loop?

Loop back? NO. If the line is long enough to need termination (longer than risetime*c/10 or so), then drive it strongly enough to end terminate it properly, and match the line and termination reasonably well. If the line is short enough not to need termination, then it won't need the 'extra' conductivity that you seem to be looking for with the looped back connection.

A series resistor? NO. That style of source-end termination only works for a single point receiver at the end of the line. At earlier points on the line, you get the worst possible waveform for your receivers, which is a step to half voltage, followed by a dwell, followed by another step to full voltage.

The problem I see with the loop topology is that it's hard to place the terminators correctly to suppress reflections. this doesn't matter much at 8MHz, but might have become an issue at higher frequencies or with longer lines.

The only case I see where the dotted lines could help is when the propagation delay from master to SLAVE 6 was unacceptable. In that case, you would connect the dotted lines, but break the loop somewhere in the middle (say, between SLAVE 3 and SLAVE 4), and terminate each branch independently.

With fast signals or long lines, signal reflection becomes a problem. In those cases, you have to properly terminate the line by putting a resistor connecting the end of the line and ground (not the beginning of the line). The termination makes the line "look electrically infinite", so the signal doesn't come back. Termination sometimes also needs LC elements to counter reactive impedance.

However, for your case, termination is not needed (short line for you signal speed, assuming you're not operating in an electrically noisy environment). That's why you see no difference. It is also bad design practice to loop your signal. Doing so makes the signal loop, introducing noise in your circuit (and it loops in both directions). It also makes it easier to capture RF noise in your lines.

Additionally, it's usually also bad to loop ground and power lines. Digital switching noise and ground loops are there too, and they can have dramatic impact on analog circuits. If needed, you can use Y forks (no connection in the far end) to reduce voltage drop. If that's not enough, use wider traces (wider trace->less resistance->less voltage drop).