Adapting an HF attenuator to higher power
You'll need 5 dB overall gain drop, so your options here seem to be the following ones:


SIMPLEST (and straightforward) OPTION): use an external 50 \$\Omega\$ 5 dB attenuator capable of handling 8W input power. Design and build it from scratch using one of the many available online calculators.
SIMPLE (but not straighforward) OPTION: Increase the input attenuation by 5 dB. This means changing the values of \$R_2\$, \$R_3\$, \$R_4\$ and \$R_5\$, and also means changing the power rating of the resistors because they will dissipate more power. The input attenuator also provides impedance matching, so you should recalculate these values taking into account \$R_8\$ and the input impedance of the MOSFET gate (according to the datasheet: 180 pF @ 1 MHz).
COMPLEX OPTION: Reduce the gain of the transistor by 5 dB (this is is also a more power-efficient option). You achieve this by changing the P1 potentiometer setting so that the MOSFET gate voltage is lower and the bias current is reduced. You can adjust the gain to a lower setting during the tuning process. In this case you don't have to change the values of \$R_2\$, \$R_3\$, \$R_4\$ and \$R_5\$, but you must change their power rating along with the power rating of \$R_8\$ and the current rating of \$L_1\$. You must also check that the gate-source voltage doesn't exceed the +/20V voltage rating of MOSFET. At 8W input, this a very real possibility and could mean this option is not feasible.
EVEN MORE COMPLEX (but possibly optimal) OPTION: A combination of option 2 and 3. Split the 5 dB gain drop between the input attenuator and the transistor gain itself.


For reference purposes: IRF510 datasheet.