I guess we could have different type of degrees when referring to a sleeper, for instant I saw civic type r fn2 or 3L BMW, being beaten by CL9 cars with just reflash and exhaust headers, this kind of sleeper when no one expected such thing from a family sedan with a stock look.
If for you a sleeper means something else like something that can beat a Ferrari cars I don't know.
In the end I'm talking about what you can achieve with a decent investment before going crazy and spend money at a logarithmic rate.
Don't look only at the top gain. I saw dynos with reflash I/H/E with up to 40hp on the middle.
Mid-range is very important, (look at the versus made by Hondata, civic 1.5 turbo Vs civic si with more HP. The first one wins due more mid-range power).
It's sad to see people wasting their small budget only on not so important aestethic stuff, when we are lucky to have one of the best NA engine built by Honda under the hood (but with the wrong tune from factory but at least easily fixable by a decent investment).
Don't get me wrong, I'm not against any other things like suspension, wheels and tires, k20 head, supercharger, etc, I'm just talking from the point of a certain budget, that I'll start first by fixing what was wrong from the factory, the engine tune, being the cheapest thing and most rewarding Vs money invested.
Sure if you have money for others bigger upgrades, sure you can go the Flash Pro route and so on.
'07 Euro Accord CL9 Type-S: Mugen Quick Shift, Alutec strut-bar, H&R front and rear anti-roll bars kit, A-Spec suspension, JDM Type-S front grille, Wavetrac LSD, FlashPro, littlebo's headers, Comptech Icebox, Hondata IM gasket, P2R IM gasket, J35 TB + larger TB to intake box pipe + IM port matched, Flashpro vittuned