TBI throttle body, restrictive or normal?

[mmigacz]

I have a stock 5.7 liter TBI (500 cfm) on my 351W (ford). I noticed while datalogging at WOT, that the Map values get as low as 92 kpa. When the engine is off, the map sensor reads 99 kpa. Suspecting that my air cleaner (spark arrestor on a boat) was the reason for the pressure drop, I took the air cleaner off and collected some more data at WOT. No difference.

Would it make sense to change the throttle body to a 454 throttle body (670 cfm)? This might be a waist of time, but I have a 454 throttle body to try. Any opinions?