Well, I've just spent an enjoyable half-hour playing Cloud Master - it's a fairly basic side-scroller, but is pretty playable. It kinda reminds me of an 8-bit Gynoug (aka Wizard of Wor), which is no bad thing, since that's one of my favourite Megadrive shooters.
I'll talk a bit about one of my fave upward-scrolling shoot-em-ups, Bomber Raid. lol. Bomber Raid probably has the plainest looking title screen I've seen on the Master System. Oh well never mind about that. In this game you control what looks like an old WWII plane (I did'nt read the instruction manual so I don't really know the story for the game). You fly over sea and land shooting up Planes, tanks, submarines, ships etc and fight a big boss at the end of all the 5 stages. The game is challenging and a lot of fun to play. You can continue from the begining of the level you finished on if the game ends, but its still a challenge to beat. The really jolly war time music, and other tunes that play through the game also I think is part of the reason why I like this perhaps seemingly ordinary game.
Anyhow I'd recommend a try of it to you, should the chance come about. I've got a few other games in mind to recommend aswell.
Yeah Power Strike is one of the toughest up-scrolling shoot-em ups I've played. No-one could accuse this game of being too slow. lol. It moves faster than probably any other up-scrolling shooter I've seen. And what a range of fire powers you have in this game too. I've only made it as far as Stage 4 (of 6) in this game. You are infinitely allowed to continue from the begining of any level you 'game over' on. The story of the game I think is there was some of sort of disaster in which all these mutant plants rose up to threaten the world.
Heres another decent top-scrolling shooter. Action Fighter (was originally an arcade game I understand), where you go upward in a motorbike, car and then a plane. Its really fun to watch as well as play. Theres 6 levels, each with land, air and sea bosses and its a tough game to complete (I found it so hard, that I ended up using a cheat to complete the game).