This might be the case for some specific drugs but for others it is definitely not true. For example the B.A. of oral and rectal morphine is the same with rectal producing a stronger feeling due to quicker onset, however if you increase the pH of the rectal environment you can almost double the B.A. of the morphine in addition to the better onset.^ I remember once being admonished by Captain Heroin for stating that rectal BA was higher than oral. He stated (he didn't cite a source, and I just took him for his word as his point was more than plausible) that BAs are the same, and that the only difference between the two is that plugging has a faster onset, therefore feels stronger.
Oral BA 20%. I've never seen a value for rectal but there is a large discrepancy between PO and parenteral so likely quite large first pass metabolism which you will (hopefully) be bypassing. I'd say I personally noticed a 2-3 fold increase when administered rectally in a slightly alkaline solution.