All joking aisde, I'm looking ain a postage stamp sized window of your photo (at work!
) - is your transom flat across the top? If so, Beamishken's suggestion isn't so daft...... (and before you all go down the "twin vs single" arguments let's not start that all again here....
Also dinae fasch yersel' - pretty much everyone on here would say 40 HP minimum for an SR4. I drove mine around with a 25 on the back for years and happily cruised at 20 knots. I now have a more suitably matched engine & boat....and still cruise around at 20 odd knots! Granted at 4500rpm rather than the 5500 I used to.....
Assuming it's power trim, have a play with that - you might eek another couple of knots (or reduce the revs a bit) by trimming it right.
Moving the weight around (anchor / fuel etc) might also help - the less Hp you have the more critical that becomes.